Does the "must approve users" option really result in revoke access of previous members?

somehow, i missed this answer. thanks for the answer.

just to be sure; this means, if in our live forum i activate the “must approve users”; it does not delete previous user’s access to our site (previous sign ups). is it correct?

what will happen to those users then?

in our invite-only forum, we need a request an invitation button. and i guess this option can do the job. is it correct?