Robin, I will mention one thing I have experienced in playing in an ensemble group of varying levels and I hope it will be helpful in having success with your group.
In the group I was a part of a number of years ago, at least half were more experienced in reading than I was and had played many of the pieces that were chosen for years. The pieces were all new to me and I was an inexperienced sight reader. I was unable to partipate without a lot of stress when we were expected to play a piece at first sight. I did not have the experience to be able to just play what I could and keep track of the place we were in the music. I asked if we could decide before our meeting
what pieces we would be playing and which parts we would play so those of us who were not good readers would be able to fully participate. After our parts were assigned, I took the pieces home and practiced my part diligently. When I went to the next meeting, thinking I was well prepared, the tempo they chose to play exceeded that which I had been practicing ! There was no indication of tempo on the scores I had and the group, without conductor, would often speed up on slow pieces so that everything was played at a similar tempo even if it *was* marked largo or andante or presto . I found this to be unbearably frustrating.
So the short of it all... when you give people a piece to play, give a bpm target at which they should practice for the next rehearsal. That will probably help the less experienced members a great deal.
When the sun shines, bask.
Classical Guitar forever!