multisite user admin

[Copied across from the main forum, as I'm getting no replies there - this might be a better home]

I have set up a multi-site, and would like each separate site to have a nominated user that can approve new user requests, or create them, for their own (sub-)site.

Is this possible? I have searched for every relevant phrase I can think of but am getting nowhere.

Any suggestions where I might go for a solution, or a work-around?

Thanks in advance!