Yes, it is sort of possible.
What a lot of them do is just aggro a mob, and bring it to the group. If the group is full of people who don't know, or fighting mulitple mobs, the there will be someone that kicks off an AOE. All of a sudden, you just aquired the new mob.
Is this allowed? No Does it happen? many, many times, if you get in a farmer's way.
And most of the time, they will keep on doing it until either you leave, or they get reported and removed.