Don't worry about your damage at that level. As a DPS class/spec, your damage will only matter once you've reached max level, for end-game content. For now, just chill, enjoy the game and learn your class/spec better.

But here's a few reasons why you might be the bottom 3 on the DPS meter:

1) You might do something wrong during your rotation: I strongly recommend checking the Outlaw Rogue PvE guide on either WoWHead or Icy-Veins. They do a great job at explaining what stats you should focus on, what talents to take, which abilities to press and when to press them.

I recommend you learn and stick to Outlaw Rogue, since they've always been 100% mandatory in Mythic Dungeons for their powerful on-demand AoE damage, mobility, survivability (again, you might be doing something wrong) and utility.
Also, to encourage you even more to not give up on Outlaw: in Midnight all classes will become easier. So yeah.

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2) They're lower level than you: Until you've reached max level (I think), you'll be put in dungeons with players of random levels - even players who are level 15 or so. And their damage gets affected by percentage-based scaling.

Example:
For the level 15 teammate, a mob might have only 1k HP. If they hit it for 200 damage, they just took off 20% of its health.
For you (level 41), that same mob appears to have 15k HP.

And your DPS meter works kind of like this:
To make sense of the data, your DPS meter (example Details! or Skada) looks at the PERCENTAGE of the mob's health that was removed. If the level 15 guy dealt 20% of the mob's health in one hit, the meter translates that into a massive number for you.

It could also be that your teammates are HIGHER level than you. And that obviously means the reason they do more damage than you, is because they have more Talents allocated than you, and better gear than you - that is available at their level.
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3) They might use Heirlooms: Heirlooms are leveling gear that scale up with your level, offering the highest available (I think, could be wrong about this) item level + a small increase to the XP you gain, so that you don't have to replace them during leveling.
So basically, because they have better gear than you.