I think that just as in Eve how a "guild" is called a corporation, I think it would be appropriate for "guilds" in STO to be called Fleets
Now to answer a previous question about you simply being with other individuals and not part of any particular fleet. I can see it being organized like this.
1 person = Star Fleet Officer
2 to 8 persons = Away team
1 - 75+ Persons = Fleet
Now the way I have worked this out is not unlike many other MMO's today where a single individual you would simply be by your self or soloing.
Should you decide to join a small group to go and raid or travel to complete some specialized task, you would consist of a team made up of 2 - 8 individuals, the cannon refers to this form of grouping as a Away Team, so why not simply carry that over to STO.
And of course the organizational structure that one might create themselves and later join with a multitude of other players is known as a Fleet