It's a nice idea and I promoted. However, I suspect that in terms of performance it may be a challenge to implement.

You can do a @DBLookup of said group name and populate into a Computed for Display field then, have your field do an @Contains on that field to speed-up performance. Or even better, Do both in one line of code (e.g. @Contains(<user_name>; in @dblookup("group_name")))and store the result in the CFD field as a boolean value in a text-field.

Then refer to this field for your access needs while inside the document.

Looks to me you can do this already using:
@UserNamesList = "Group"
(ps: don't use @ismember)

I bow to Theo's expertise.

Form help "@UserNamesList returns a text list containing the following information for the current user"
Emphasis on "current user"!
So you can't check anything else than current user membership to specific group/groups with this @UserNamesList = "Group" technique.

Yes. Sounds like a nice option to me.

Actually why just @isMeemberOf...

Maybe same function but with parameter @UserNamesList("username/group"). Obvious return same results as normal @UserNamesList but we can specify the user or group.

Then we can process is as we like.

for my usecase the @userNameList is enough. Thanks Teo for the tip, you learn something new every day...

Oups: Theo! Sorry!