You could spend the entire game playing alone if you'd like and there's plenty of content to keep you happy for a long time. If you want to try your luck at some of the harder content though, you'll need to group up with other players.

Group sizes can vary between 2 and 16 players, though anything above 4 will be limited to Operations and PvP Warzones. If your group is under 4 players, your companions may be used to fill in the slots. It's usually a good idea to use a companion to fill a role that is not present in the group. If you have a tank and a healer, then get a companion that's purely for damage. Organized groups, past level 15, should have 1 tank, 1 healer and 2 DPS. Operations will require different group makeups depending on their content but usually the ratio will remain.

Understanding how to play your class is one thing; knowing how to play your role is another. A tank that only focuses on themselves will get their group killed, just like a healer that can't work an emergency.

How Each Role Act in Party

Tank Role
There are 2 main goals here, keep the healer alive and avoid the stuff you can avoid. If the DPS die, there are very high odds that it was their fault and nearly any content can be completed by a healer and a tank, with time being the largest factor for success.

Tanks operate with the mechanics of threat and defense. Threat is a measure of enemy hate and if you have the most threat, they will attack you. You can force enemies to attack you through taunts but mostly just through using your skills you should have enough threat to keep a target attacking you. Keep an eye on your abilities that increase your threat level as they should be used to keep on top of your threat level.

Defenses come in two main types, the actual defense which makes an enemy miss you (dodge, parry or an outright miss) - called avoidance - and shielding and armor - called damage mitigation. Every tank has armor and every tank can defend but not all of them can use a shield. Using defensive cooldown abilities can keep you alive through a heavy enemy attack or at the start of a group fight, where multiple people are attacking you.
The second goal for a tank is avoiding the avoidable attacks. Often times these are area effects on the ground that have visual cues, though sometimes it has to deal with enemies who knockdown. A good idea is to keep your back against a wall or structure so that if you are knocked down, you don't go flying into more bad things and you won't have to run to get back to the enemy.

In a group one of the particular tasks a tank will have is the pull, or the act of starting combat. On bosses this is usually pretty simple as there is but one target. In group combat however, you need to pick the appropriate enemy to kill first and 99% of the time, that's the group healer. They can make everyone's life a lot harder. So, jump in with a direct attack on the healer and then follow with an AE attack to get the rest of the group to attack you. Then it's a balance of keeping threat on all targets while ensuring that your primary target is unable to act. In terms of target priority, you want to kill healers, melee DPS, ranged DPS and finally tanks.

Damage Role
The two goals for a damage player are keeping the healer alive and avoiding what can be avoided. Plainly and simply, a dead healer will mean death for you and the group in short order. Even though the tank will try to accomplish the same goal, sometimes there are enemies that will bypass the tank and head straight for the healer. Examples include bosses that summon additional reinforcements, most of the time they will head straight to the healer. Your role is to take them out quickly.

Secondly, your role is to not get hit. Many enemies have frontal attacks, meaning you want to keep your distance from their faces. Melee should stand to their sides while ranged characters should stay close to maximum range. When you see something on the ground that deals damage, typically a reticule before the actual attack, move, even if this means you aren't dealing damage. The adage goes, a dead DPS does no DPS.

In some cases, there is damage that you simply cannot avoid. These include AE attacks that have no minimum range, boss attacks that hit random players and simple bad timing. These attacks are in the minority however when you look at the total amount of attacks happening in a group. If you are getting hit against groups of enemies and it's not AE damage, you're doing something wrong. You should, whenever possible, be targeting the same enemy as the tank is targeting. This will ensure that you are competing with the tank's threat level rather than the tank not having any threat at all on your target. In many cases this means holding off on your attacks for a second or two to ensure that the tank has the enemy's attention and then you can start.

If you make a mistake and an enemy is attacking you don't continue damaging them. Instead you can use a defensive ability, such as a stun or knockdown, giving the tank time to pick them up. Basically, if you're attacking the same target as the tank you won't die unless the tank dies. In 99% of the cases where a DPS dies and the tank does not, the DPS has made the mistake. It isn't all run and gun.

When attacking an enemy with activation times or channel times you want to be interrupting them when possible, especially the healers. Stuns and knockdowns will also work well. The difference between hitting a target and letting them get a heal off is that normally that heal is more powerful than any damage you can do, especially on bosses. A 30% heal on a boss can undo 2-3 minutes of work in mere seconds.

Healing Role
By far the most important role, the healer is all about keeping the tank and the group alive. Your two goals are simple. Tanks is alive and you're avoiding the stuff you can avoid. Healing the DPS is really an afterthought once the first two are complete as a group made up of only DPS has next to no chance of success when compared to any type of group makeup with a healer.

The concept of triage applies in TOR, where you use the appropriate heal for the appropriate moment. This situation exists because every healer has a limited energy pool that cannot be increased with additional stats. All healers will come with a quick heal, a strong heal and heal over time and an AE heal in their basic toolkit. Some healers will have additional choices along the way, perhaps extra types of heals or shields. Understanding how they all work together is key.

Your first step is setting up the tank's defenses. If you have an ability that can protect the tank, it should be active at all times possible. Any damage they don't take is damage you don't have to heal. Secondly, if you have a HoT ability available, it should be active on the tank while they are taking damage. These typically have the highest return of health healed based on energy cost, plus they tend to be set-it-and-forget-it. These abilities also work well on DPS that take minor damage and it's unlikely that they will continue to take damage before the HoT has completed. Your slow and strong heal should be used on the tank for most events as they are going to take the most damage. Many classes have abilities that increase either your power or critical chance and these are best teamed with your strong heal for maximum effect.

Finally, you need to stay alive as well and this usually means moving out of bad stuff on the ground. Many times there will be an effect on the ground indicating an upcoming attack and this is your cue to move. Unless the tank is going to die without your heal, you should be stopping what you are doing and moving to a safe location. Good news for healers is that they all operate from range, meaning that you can heal from pretty much anywhere in a fight as long as you can see the other players. This gives you an advantage of perhaps standing on something at a different level, or behind a box while the enemy can't directly see you and hopefully avoid any random attack they might have.