See attached file for mission example.
1.Basically, I've put a four man unit outside of the building. Player is NOT the commander.
2. Add waypoints on the building. I've put position # 8, which is directly in front of the stairs, then position # 11, which is on the second floor right up the stairs, then position # 24, which is on the last floor.
Follow the AI commander and check his behavior. He'll go to position # 8, then move to where position # 11 is on the second floor, but stays beneath on the first floor.
In the mission file I attached, I also include another four-men team, with the commander positioned in the stairs. He'll go up as expected to his waypoints, but the other team members will roam on the first floor, never getting to the stairs.