I have set up a Multisite to allow logged in users to create their own sites. The default Wordpress setting is users can create unlimited number of sites. However, if I want free (non-paid) users to be able to create 1 site and paid users to be able to create 3 sites, I don't find any way to set these limits. I use both Membership Plugin and PRO Sites. The only plugin I know of that have this feature is S2Member, but I don't want to mix S2Member with WPMU plugins, because overlap in other features will make it very difficult to get everything working right.