Is your group of players a mix of f2p and subscribers? If so that maybe preventing you from creating the guild. If you are f2p and have subscribers in the group that maybe the problem since f2p can only be the leader of a f2p guild.
I have not tried this listed above but that is my understanding of how the guild options works.
I am pretty sure it works that way also.
A F2P may only be the leader of an ALL F2P guild...HOWEVER F2P players CAN join a subscriber guild. They just can not have underlings that sub.