It appears that at some point in time I renamed the global project, whatever that is, to something else (say, "X") because I wanted to use it.
I haven't been using it at all for some time, but today I finally decided to create an issue. It went fine, but I couldn't assign a user because there were no users in this project's team. And apparently, it's impossible to add users to the global project.
I tried to rename the project from X to Global. All was fine, except I couldn't create a new project X because YouTrack thought it had a duplicate name. "OK", I thought, "the name must still be used somewhere internally, so I can just remove the project Global". And I did.
The only problem (or at least, the only one that I've found) is that now all groups that were mentioning the project X still do mention X even though it was renamed and even removed.
I was able to change all relevant groups' projects to a dummy temporary project, except for the "All Users" group - it still mentions project X. Because of that, I cannot create a project named X.
What should I do in order to have a genuine project X that's not some global project, so I can finally add people to it?
Also, the documentation does not appear to be mentioning the Global Project in any way.