I have moved my sites over to a new host as my current one

I have moved my sites over to a new host as my current one doesn't support WPMU. I am redoing all the sites from the ground up. I haven't moved the domain over yet. I need to figure out how to have all three sites share users. I know there are plugins, but which one's are good? I have the main site, and another learning site and a community site where i have bbpress and buddypress (I know, I am using bbpress and buddypress on a secondary site, not on the main). What's the best way to make sure I am setting up my site so they can all share users? I want them to be able to sign up on any three and share... will buddypress and bbpress mess that up? Thanks.