Can I use one membership for different domains?

Hello, I create my own theme and I want to use it on all my 3 multisite domains + subdomains . I want to use the same idea like themeforest they use 7 domains under one theme so it looking just like one website . my problem is the membership because in themeforest you register in any domain and directly ( auto ) your membership is access in all the 7 domains . their membership is one for various domains , Can I use your membership2 to do that? and how to do that in details please ?

second question please ==> When I use your membership 2 and buddy-press I have 2 pages for register and 2 pages for activate what should I do ?

Thanks you