Re: Several Buddypress sites one login?
I assume that you will have mutliple domains, each with their own BuddyPress install. If that is the case, there is no easy solution available. What you are asking for is a single-sign on (SSO) solution between multiple domains that each have their own BP instance. This is beyond the scope of a single WPMU install with a domain mapper. This is not an easy thing to do by yourself and I’m afraid it will not be possible at this time without some heavy custom coding on your part.
SSO is a hot topic with several technologies and standards trying to work their way into acceptance and use. The best-known of these is currently OpenID.
There is a WordPress plugin that allows you to integrate OpenID sign ons within a WordPress site, but it is out of date. With both Chris Messina and Will Norris having recently been hired by Google, I’m not sure if this plugin will be updated any time soon. I suggest contacting the Diso Project and inquire about the status of their WordPress efforts.
Also, there is talk of implementing the Open Stack in a future version of BuddyPress. But this will not be anytime soon.
Another possibility, which in my opinion is the best solution for facilitating federated identity management across the Web, is FOAF+SSL as a WebID. But that solution is even further off at this stage as it requires individuals to implement their own WebID. OpenID is a 3rd-party identity provider whereas a properly-implemented WebID is a 1st-party solution that each individual owns and controls.