This, you really do benefit from helping somebody out in this game. If the mob tag is purple then they have tagged it and you can also earn XP by helping out! I do this all the time and I've not had anybody moan yet :P You can also help in the opposite manner, if somebody starts attacking your mob and it's at 50%, I often leave the mob for them to finish off so they can do enough damage to also get some XP for it.
Also, high five for asking! It's very refreshing to have a new online gamer asking how to address things in the community and not being a complete ass! Welcome!
