[itsme]

It's up to the user to create his own group "Hidden". Then using "Exclude" "Hidden" in the filters allows to do already what such a special group would do.
Using "Unassigned" / "Assigned" (Zugeordnet / Nicht zugeordnet) might be another option. It allows selecting songs that are assigned / not assigned to any of the groups.
I use it often for "Collections": I have collections for every band or regular session plus some extra collections like "ToDo", "to be practiced" and so on.
"Unassigned" for "Collections" finds all songs that are not in a collection, the currently unused ones. "Assigned" for "Collections" finds the used ones. "Exclude" can be applied on meta groups for placeholders, table of content, that you could create for files that you have in MSP that are not songs.
I prefer using the existing filter mechanisms over creating a special group that is in fact nothing different. They are powerful and flexible enough.
Persisting the filters when MSP is closed and reopened might be convenient, but is not essential. I rarely shut down my tablet and keep MSP open most of the time. The sleep modes of Android do a pretty fine job.