Correct that faction and level won't matter. But as far as quests, you can't retake quests that you have already done. You can quest together in an area where none of you have completed the quests, or you can just follow your friends along and help them to do theirs.
You can help them do the quests, but you won't get any reward from it except the basic stuff for killing enemies. And it might be a bit weird - they'll be talking to NPCs you can't see and things like that.
PC EU player | She/her/hers | PAWS (Positively Against Wrip-off Stuff) - Say No to Crown Crates!
"Remember in this game we call life that no one said it's fair"