We have begun playing the guild expedition in a guild with only 2 members. We have progressed to the 2nd level, paid to go on, and have begun to proceed there. Seems to be working fine so far. I assume it will also work for a single member guild. It seems the only criteria is for there to be a guild.
The nice thing about it is that each of our stats show up in our respective PvP areas and accumulate there as well as building the guild's standing. I don't need to attack and plunder my neighbors to do the PvP, nor need do the GvG either. I can work for the guild and help myself all at one time without getting too involved.