1)In Group expert Dialog Select Options

2) In Change Group Options Dialog select Options Tab

3) In Options Tab Check Customize Group Name Field

4) Once you done you can choose from existing field so if you have manager name field in table select that or else Use a Formula as Group Name where you can add the Manager Name as per the grpmem.manager_flag  field value

 

maybe something like below

 

If grpmem.manager_flag  = 1 then Managername A

else if  ......

 

or you use Select Case

 

Cheers

Rahul