There is lag/delay in gaining the target of the toon you want to /assist.
This is pretty common even in WoW.

What moosejaw said about setting your follower's assist key to the same one your tank pulls with works great and gives your tank a little lead to gain aggro.

And I know for sure you at least have to be in a group to /follow. I haven't tested a raid though. :P

Ok..I'm really really logging in to play now... :whistling: