I use two buttons to handle the phasing issues

1) /script LeaveParty();

2) Just a basic invite macro for each 4 party group (On the first group I use 2 macros one to form a party and the other to form a raid with all 8 if I need to get them back on the same phase...)

That way with just a couple button presses your toons will all leave group then be re-invited..

Questing with 8 is really nice the majority of quests you only need to do one time... all 8 get kill credit for most quests.. if you go up to 10+ then you end up having to do many quests more than once so they all get credit..