Allow all members of a team to modify a Case that owned by a System User

Hi all,

Using CRM 2013.. Soon to be 2015...

The requirement is as follows:

• Users of Role "Foo" will always be members of multiple Teams
• Users of Role "Foo" should be able to view & modify Cases that are Owned by other Team members

Jim and Bob both have the "Foo" Role and are both members of Team "Bar".  Jim has been assigned a Case; he is the Owner of the Case.  

How can I allow Bob to view and modify Jim's Case without Jim actively "Sharing" his Case?

One idea I had is to use Access Teams and to never assign a Case directly to a System User.  Problem is, all user's belong to the same Business Unit, which is essentially the Organization, and the permissions system in CRM doesn't appear to be granular enough to grant privileges at Team level - it's either User or Business Unit.

We're in our first Sprint here on our first CRM project, so apologies if I've messed any terminology.

Thanks,

One Struggling Team Lead (OSTL)