OK, I suspect this is a technical problem linked to the phasing server. So if you are at a certain phase of the game you cannot group with a person in a different phase. I.e. you cannot group and be in a dungeon with a person who has already completed the dungeon.
Trouble is there is no way of identifying who you can group with and who you can't. It's a free for all and a frustrating mess.
Or, am I getting this wrong and is there some way to group successfully in this game?