Generally a nonprofit may refuse membership to anyone, but your bylaws could undercut your ability to do so.  Some organizations simply say that persons who meet certain criteria and pay the annual dues are automatically members.  In that case, you may be precluded from refusing the membership.  And it may be difficult, under your state nonprofit corporation law, to expel members once admitted.  We have always thought the bylaws should specifically require a procedure to affirmatively admit members in order to preclude being caught by your own rules.  (See Ready Reference Page:  “Bylaws Function as ‘Constitution’ of Nonprofit Corporations”)

If your organization is large enough to be considered a “public accommodation” under civil rights laws, you may not be able to reject persons because they are members of a protected class, but lack of support for the mission would not likely be an illegal reason.  Even on issues involving a protected class, however, the applicability of civil rights laws is not entirely clear.  The Jaycees were prohibited from denying membership to women under state civil rights laws, but the U.S. Supreme Court has held that the Boy Scouts are permitted to ban gays and atheists within their First Amendment right of freedom of expression.