The music for the original version of the Beggar's Opera consisted of numerous popular songs of the day. Because of modern copyright considerations, we couldn't do that - so we have written our own music to the new lyrics for the show.
In order to do this, we had to have an overall concept of who the show was meant to be performed by, and we took as our model a minimal cast who remain on stage throughout, filling in the minor parts and acting as backing vocalists when required.
In addition, we decided that we would use a trio of female vocalists (who also take minor roles in the piece) as a sort of backing group/Greek chorus who comment on the action throughout.
We also wanted to use dancers in some of the numbers to add to the effects we were trying to create, but because of the additional rehearsal time required for this, we decided that we would try to pre-record the dancers on video, and to project them onto large screens above the stage. This has the advantage of requiring a smaller cast of 'live' actors, and removes the distraction of stage noise during the dance sequences.
We also decided that we would keep the arrangements as simple as we could, so that the show could be performed with a minimal group of musicians to keep band costs to a minimum.
So how did we go about writing the music? Well, it wasn't a question of sitting in a room together and bouncing ideas around. We don't live that near each other that we can pop round for a quick rehearsal session, so we used Sibelius to write down the initial ideas, then distribute it using Dropbox. Then one of us would record a basic version, and distribute it (again via Dropbox) so that the others could add parts of their own. That way we eventually ended up with a basic track that we could add the vocals to.