i consider it solved after read this http://buddypress.org/support/topic/multi-site-multi-network-questions/
this thread answer my question.
The user can login to any network using username and the password using theirs at main blog.
But, the avatar like mercime said. its only per site. User HAVE TO upload avatar again on each network.
(3.5.1 , 1.7.1
I had experienced that my user registered in Multisite Subdir who already upload avatar in first blog (Main Blog), but the avatar not showing up in second blog.
I already made BP network activated.
How to fix ? is it a bug ?
@afjsystem active 2 weeks ago