Re: Sharing user/login with buddypress and other wordpress sites
@frizzo – sounds like you are looking to implement WPMU (multisite) 
Here is a link to help get you started: https://codex.wordpress.org/Create_A_Network
The way I’m doing it, our main buddypress site is the master domain, all users from that site can use their accounts around any of our sub-blogs or domains. There’s even a premium plugin for domain name support so you’re not just limited to a subdirectory.
Hope this helps!
