How to define workgroups and assign it into each member?

Does anyone have any suggestions on how to define workgroups and assign it
into each member? In office, there are 5 Departments, Sales, Marketing, IT,
Finance, and Admin. And I would like to define a workgroup for each
department, after that, I would like to assign access right to each staff, if
the staff is Admin, then they can only access admin folder. Some member could
belong to 2 groups, therefore, they have both access right within this 2
groups.
Does anyone have any suggestions?
Thanks in advance for any suggestions
Eric