Jane - yes, of course you can simply decline unwanted invitations but it's really not as simple as that on a phone and it does take up lots of space on an already crowded screen as well as time to scroll through the irrelevant stuff to reach the relevant. I ignored it all and decided to wait until I got to my computer ... and then used a discussion topic to flag up my concern. But I can see how someone else, equally "getting to grips with the peculiarities of how the group/invitation thing works", might have not realised that deleting something for him/herself might affect the whole page for everyone.
A bit of tolerance on both sides is a good thing, it seems to me. And thank you, Vic, for taking my comments entirely in the spirit in which they were meant!