Multi Site Set up and Plugins to use

I currently have a multisite set up and need to have a single registration on the main page. Would like to have users then have access to any of the sub domains ( I understand that Buddypress is only available on the Network Admin Main Site and not able to integrate into subdomains via multisite?

Any suggestions of plugins to use as each sub domain will ideally have their own “social community” but would only like to have one Login. Thanks in advance.