If you don't need PFS you could probably modify MLS to be a bit simpler. Firstly, no need for sender keys. A single group key will do. When you remove someone

If you don't need PFS you could probably modify MLS to be a bit simpler. Firstly, no need for sender keys. A single group key will do. When you remove someone from the group, you still do the group update and derive a new group key (unless you don't want post-compromise security either, in which case you can throw out the entire tree concept). Now when you add someone to the group, you share with them the entire historical group transcript along with all the previous group keys. Is this roughly what you're looking for?