Create mini-site community with Craft
There is no plugin that provides this type of functionality but there really isn't any need for one either. Depending on how many pages their mini-sites needs to be, this really can all be handled with front-end entry forms and how complicated it can get is entirely up to you. 

You should create their control-panel's using routes (possibly not even necessary depending on how you structure it). There you'd allow for any of the entry forms, etc. 

In order to give more advice on how to go about this we'd really need more detail on exactly what each of these multi-sites needs to be capable of but this just seems like a standard site allowing unique front-end entry creation which is all able to be done without the use of any plugins, even while allowing it to get quite advanced and complicated.