Note: On perusing several user group home pages, I find that one page will often have several of the following: user group, user-group, users group, users' group, user's group. I encourage WWW authors to be consistant to avoid ruffling the fur of consistency hounds.
I find "users' group" most natural, but I suggest "user group" for the following reasons: 1) Some groups don't seem quite accurately named if a possessive form is used, as in a mythical "Red Hat User's Group", when the organization belongs to Red Hat rather than the users. 2) When said, there is no ambiguity whether "user's group" or "users' group" is meant. 3) Replacing "user" with something inanimate like "tree" sounds best as "tree group".