Business Units are handy if there is a need to impose data access restrictions. There are cases however that a user belonging to a business unit may still need to have access to records belonging to the other business units.
Case:
User A belong to Business Unit A
User B belongs to Business Unit B
User A needs to have access to records owned by users in Business Unit B.
What should I do?
1. Set up a Team in Business Unit B.
2. Make User A a member of that team.
3. Assign a security role to the team.
– If the team’s access-level privilege for an entity (e.g. Account) is USER only, then User A will be able to access the records owned by or shared to the TEAM.
– If the team’s access-level privilege for an entity (e.g. Account) is BUSINESS UNIT, then User A will be able to access the records within the BUSINESS UNIT of the team (i.e. Businee Unit B).
Key Point to Remember:
A user’s record and access level privileges is the sum of the user’s own security role + the security role of all the teams the user is a member of.
Striving to Make Things Count 
It’s funny I came across this post which is exactly the same concept I am trying to understand right now.
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