Add users on multiple databases when someone registers on main site

My client has 4 scripts installed on his server. 1 Main Site has dedicated cPanel account, while 3 other sites are sharing 1 single cPanel account (2 are added as subdomains) But they're all being hosted on same dedicated server. What my client actually wants is, 1. when someone registers on 'Main Site', they should automatically be registered to the other 3 sites also. 2. From the 3 other sites, all members can access the 1st site, but to have it's premium features, they need to have a premium membership and remaining 2 sites should only be access if the member has Premium Membership on 'Main Site' If they try to access those 3 sites without being a Premium Member on main site, it should redirect them to the page I'll provide. Please send your proposal only if you understood what my client wants and if you can get the job done. So, if you can do this job, I might ask you to do the registration part first, if you've done it, then I'll accept your proposal and pay the money when you finish the cron job of Premium Membership thing.