Assigning Privileges to a Team

Hi,

I have users in a single Business Unit, where they are then split into Teams within the Business Unit.  I have given the users write access to their records only - set privilege at User.  I now want to allow members of each team to have write access to all records within the team they are a member of. I have set up a new security role which I was planning on assigning to the team,  however the permissions only allow me to set write access at User, then Business Unit. Can anyone advise how I can/if I can set this overriding permission at a team level?

Thanks in advance.

Agnes