Ordinarily, a nonprofit membership corporation may select its own members and may refuse to admit someone whom the selection committee feels would not fit in.  Our model bylaws for membership nonprofits includes a specific statement that membership may be denied for any reason deemed sufficient to the Directors.  (See Ready Reference Page:  “Bylaws Function as ‘Constitution’ of Nonprofit Corporations.”)

If the organization is so large and open that it is categorized as a “public accommodation” under anti-discrimination laws, it may not be able to discriminate against members of a protected class, and “you don’t fit in” may be considered code for such discrimination.  But if it is a decision legitimately based on personality alone, it may not be discriminatory and not illegal.