Service Level Management

I am confused as how to best set up our SLA's. We have a dozen or more SLA's for departments on our campus.

They have two response levels, 1- Immediate response with 24 hour resolution

                                              2- 1-3 hour response with 60 hour resolution

I am trying to figure out how best to enter this in Service Desk.

Each department has some people on Level 1 support and some on Level 2. I need to figure out if I should just create 2 groups (a level 1 and level 2) and add the people to the groups, which would mix the departments.

Or should I create a group for each department and somehow determine if they are a level 1 or 2.

I have looked through the documentation and I know how to add the levels, rules, etc. I am just not aware of the best and most efficient way to set it up in Service Desk.