How to share a group folder

Version 19.0.4. I have one main folder with one sub-folder each per client. The group of graphic designers has access to that main folder - hence, to all clients’ sub-folders as well. For those clients that must have access to their files, I created one group each and added that particular client. I then assigned “their” sub-folder as group folder. This worked fine for a while. I believe I still had to use the sharing fork, but it worked. But now, I cannot replicate.

I am quite sure that, after hitting the fork, the menu on the right showed an item on the bottom that showed me all users from which I could select the client and set his rights. Those that worked showed a fork next to the folder, and in the share info on the right in the folder, it showed the user’s avatar.

What do I do wrong?