Oh groups should definitely be allowed! Even guilds would be
welcome by me, and I'll happily play along with every player I see out there.
What I absolutely do
not want to see, ever, is being in a guild (or affiliated to one) become a necessity.
Players should be free to roam, explore, fight, trade, do whatever they wish, as they wish. Guilds make that impossible in some games, because in their minds it leads to promotion of the group above that of the individual.
Leaders bask in the glory of their guilds once they have made a name for themselves, they gain followers and there is a very good chance that eventually it descends into tribal / thug behavior.
I have seen it in game after game after game. Not all groups do this of course, but it happens more than not.
The way to mitigate this, of course, is with space. Elite will have heaps of that, and so guilds will find their ability to influence anything, well, impotent. They will need huge numbers of people patrolling every system they want to lay claim to, all the time - and the game mechanics simply won't allow that to happen. Get a huge group of guild members together, and there simply won't be any room in their instance for them to get anyone to fight, control, persuade, ignore, laugh at or insult. It will be Guild vs NPC's - and I can't imagine any serious PvP player being satisfied with that!
This is going to be a very, very interesting game
