You can join a 1v1 fight and assist your sidenoder in defeating the enemy’s one, which is known as the “plus one” or quickly “decap” an enemy node (turn it neutral). Keep in mind that going for a full cap is usually pointless as you will struggle to defend the node vs an enemy sidenoder.
Your main damage comes from Backstab. You gain access to this when you auto-attack someone while in Stealth.
You can also use Heartseeker when the enemy is below 50% and 25% health as it will do significantly more damage in those cases.
A large part of thief’s value within team fights is the ability to interrupt enemy revive skills (for example Signet of Mercy and Glyph of Renewal ). In most cases Head Shot will be good enough to do so, however guardian has access to Stability, in which case you may need to use [skill 55031] instead.
Head Shot is also useful when assisting 1v1 or jumping on a target. You want to interrupt an important skill on the enemy (for example Heal skill).
Your best defense is to run away and focus on other objectives because your role is not to duel but to assist. That being said, you can find yourself pinned down or under pressure, in those cases you can use either Blinding Powder or Shadowstep as stunbreaks, Dagger Storm to deal with burst damage or pressure, or when waiting for Shadowstep cooldown so you can run away.
And of course, you can use Hide in Shadows for Stealth and healing, but be mindful of long cast time, use terrain as a cover.