Pre-moderation of collective blog members

I think at a certain moment there will be a problem of moderating collective blogs, in order to find some individuals there. Now it is possible to exclude only already joined people.

I want to be able to put the blog in the join mode "on request". That is, so that the superintendent can decide whether a haber man has the right to be a member of the blog, and, in particular, to see the sub-records, or not.

Technique: in this mode, a person sees the button does not “join”, but (say) “I want to participate.” After that, the caretaker is sent a letter with information about the request for participation, and in the admin part of the blog management, he sees the person in the "willing to participate" section.
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Thank. :)