In 2.10b, when I right click on an entry to add it to the group, I see a tree showing the existing groups, but when I click on one of them and then click add, nothing happens (when I save the .bib file, I see that it hasn't been added).
Adding via drag'n'drop still works. Meanwhile, We are working to get the popup running, too.
Morten Omholt Alver
This is strange. The function doesn't work because the relevant code is missing. Since I've switched computers since working on this, I dug out the files from my own computer, and there the code is complete. My best guess is that I never managed to push this code to the server. I'll update the beta branch shortly.