1 single company - comprising of separate Groups by accounts(departments..). There can be common resources working on both the Accounts (part of both the groups) we will called them as "Shared Resources", this feature works fine by having User be part of the 2 groups (Home Group A and Other Group B)
However there can also be a situation where there are resources who are meant to work EXCLUSIVELY for 1 specific Group (User's profile comprise of only 1 GROUP i.e. his Home Group), in this case, the system will still allow the select this user on a project belonging to other group (assignment of task) because this Exclusive Resource is still part of the same company as the Project Owner.
Currently there is NO SUCH option to restrict assignment of work in a situation where Users belonging to SAME COMPANY, but unique GROUPS have ZERO access to each other (creation of virtual Partition in between Users within same Companies through GROUPS).