[dreamland2.0]

The main group that is popular right now is AKB48. They have about 56 members or so now. They are divided up into 3 teams A, K and B all of about 16 girls. These three teams each perform their own 'stages' at the AKB theatre.
There is also a 'research student' team that perform at the theatre as well and also substitute for one of the girls in the AKB teams. These research students eventually get promoted to be in one of the three main teams.
For singles the most popular members are chosen to sing the A-side. The B-side is sung by 'the undergirls" who are the next most popular members.

And yes the sister groups SKE, SDN, NMB are the same concept with the three teams but they release separately and perform their own stages, etc. They are all under the same 'company' which is called AKS I think. SKE performs in Nagoya and NMB is performing in Osaka. SDN performs at the AKB theatre in Akihabara but they perform on Saturday nights for an 18+ show that's more mature than the other groups.
However all the groups have different record labels that release their music. Also each girl's promotional activities are not handled by the 'big company' but by different promotional companies.

I hope I explained it well ^^;