As long as all members of your group are below level 15, you will get a chance at a level 10 dungeon. It's not reliable though. It can be level 15 dungeon. To reliably be always at level 10, all members of your team need to be level 10 or under.

It works the same way going up. To get a level 20 dungeon, get everyone to be level 20 and below. If everyone is level 24 and below, you will sometimes get a level 20 dungeon, but it can also be a level 25 dungeon. Of course, farming at 2 levels isn't bad either... It all depends on what you want.