Currently there is a limit of eight users that can be allocated to a package/Model .
This means it's really difficult to maintain and control the users lists especially if Public Read/Write have been turned off. This can be worked around by using groups BUT even this has the eight person/group limit.
One way to address this is to allow and correctly implement groups within groups eg
Have a ReadOnly and a Write group. To allow everyone in those groups to access a Model: create an AllUser group which contains the ReadOnly and Write groups. Then set AllUser to have read access on the top level of the model.
Note: It is actually possible to do this BUT the ReadOnly and Write groups end up appearing twice in the dropdowns within PTC Integrity Modeller which is incorrect and confusing.