User’s/Users’/Users Group [duplicate]

Possible Duplicate:
“User's guide” vs. “users' guide”

If referring to a Users Group (meaning a group made up of multiple individual users who have some control of the group itself), would you use:

• A: User’s
• B: Users’
• C: Users

I have read the post on the User’s guide but in this case, plural is appropriate since the group does not belong to one individual.

Wikipedia suggests Users’ (or Users) and in my domain, a leading stats software company has a Users Group, though they then use user’s group as part of the description.

What do you think — A, B, or C?

In 'The Cambridge Guide to English Usage' Pam Peters identifies 'plural nouns which express affiliation' as one of the kinds of expression in which the apostrophe is disappearing. She gives the examples teachers college and senior citizens centre. Much will depend on the conventions in force within any particular group, but I cannot see that the apostrophe adds anything in such cases. So, for me, Users group.

If you're not going to use User Group (which is customary and which I wholeheartedly prefer), then for a group collectively owned/controlled by a plural number of users you need a plural possessive. Users’ Group.