Certainly UU congregations openly welcome atheists to their congregations as a matter of doctrine so long as they are tolerant of other members of the congregation..

Historically, UU doctrine openly professes itself to be a religion; however, the doctrine is probably the least restrictive of any religion. They openly accept members who adhere to any conception of a deity, singular or plural, popular or individual. Sermons are usually more similar to lectures and ministers act more as administrative heads or educators of the congregation than enforcers of any explicit doctrine. Further, different congregations adopt differing ceremonies, and only object to those which offend the sensibilities of the congregation. Participation can certainly replace many of the social functions that some former theists regret giving up when they left their former religious affinities, although if you have any objection to socializing with theists you might find a humanist, naturalist or skeptical organization a better fit.