BuddyPress.org

Some of the answers I’ve seen seem a little confusing on this topic and others are for WP-Mu (not available anymore)
I’m using WP 3.9.1

Say I have a general site http://www.maindomain.com
I have two subdomains group1.maindomain.com
and group2.maindomain.com
All three have a separate WP installed
I did this so both subdomains would have a separate look than each other and the main domain,
and would act as separate websites.
But the 2 subdomains will only be called from the main.
Can all three share the same members so they don’t have to register or log into each one.