When you try to assign access bundle within admin>attribute models, it shows only access bundles that have their own libraries, i.e. does not show access bundles that have no libraries.
Not entirely sure whether this is a bug or intended.
If intended, I think it would be good to allow to assign access bundles without a library as well. For example, if one wants to restrict the right to assign specific attributes only to a specific access bundle (specific users) that do not necessarily need a separate library.
Good question, I slightly updated the documentation on attribute models and access bundles to better explain the issue you are confronted with.
When you create a new group with a library, an access bundle for that group is automatically created by the system (and by default the members of that group have read rights in the group library). Conversely, for groups for which no group library was requested, no access bundle is created by default.
So in your case, you just need to create some appropriate access bundles for certain groups (or mixes of groups). Afterwards, these access bundles can be assigned to attribute models.