Hi Ian,
Welcome to the B2 forum.
Just to be clear, you have your music loaded on to your B2, you do not have any speakers directly attached to your B2, You have 4 Sonos speakers scattered round your house. You can see these 4 speakers in the B2's Web UI thus (or similar):-
B2 UI
If this is the case then your Questions
1) No The "Brennan Speakers" are there as part of the B2's internals.
2) No.
3)
How do I/Can I group my 4 speakers together? - there does not appear to be anyway of doing this.
The grouping of your Sonos speakers is achieved from the Sonos App setup (see below) not the B2 - (the B2 "reads" how the Sonos system is set up and presents this setup in the Sonos box above - you will have 4 individual Sonos speakers at the moment I suspect).
Sonos UI

See how the Sonos names are mapped across to the B2!.
The Sonos App allows you to group Sonos speakers together and give them names and also allows you to create stereo pairs (my "living room" is one such pair).
Hope this helps
Fred