My mistake was, I think, that I thought of groups as being purely a communication tool in the same way as a traditional web-based listserv (mailing list) .... well I could not have been more wrong .... and this week I woke up to the fact that the jobs I was doing with the five groups I'd formed could have all been done with one group (OK.. maybe two) providing it was correctly configured.
In fact when you set up a group in Second Life you are actually setting up a group of groups .... and each of those groups in the group (uummm ... I'm thinking of John Cleese now) can have completely different names and each of the subgroups can have completely different properties ... which in S.L. are called "Abilities".
I'm not going into all the pros, cons, whys, examples here .. but just want to make new land owners aware that they need to consider this more carefully than I did at the beginning as it not only can affect communication but also ownership of land and of objects built on that land ... also affect what different people can do on the land ... i.e. build, fly, teleport, stream in media, modify objects etc.
.... and if anyone does know of any guidance about Groups for new land owners then please leave a message in 'comments' below.