I am having SQLdata in my Employee table like below. EmpId is primary key and TName is having empty string with no value

| EmpId    | TName    |  GrId 
| -------- | -------- | ---------  
| 1        |          |  AA        
-----------------------------------
| 2        |          |  AA        
-----------------------------------
| 3        |          |  BB       
-----------------------------------
| 4        |          |  BB        
-----------------------------------
| 5        |          |  BB        
-----------------------------------

I want to auto increment the TName value based on the grouping of this column “GrId”

For EmpId = 1 and 2 we are having same GrId so we can start incremented with +1 and for EmpId=3,4,5
we are having another GrId so again start incrementing from beginning (zero) with +1

I cannot use Identity constraint on my TName column

I have tried like this but thorwing errr

Update T
Set T.TName = '('+ (select CAST(Count(*) + 1 AS Nvarchar(max)) from Employee) + ')' 
from Employee T
Where 1=1 
group by T.GrId

My expected output:

| EmpId    | TName    |  GrId 
| -------- | -------- | ---------  
| 1        |   (1)    |  AA        
-----------------------------------
| 2        |   (2)   |  AA        
-----------------------------------
| 3        |   (1)   |  BB       
-----------------------------------
| 4        |   (2)   |  BB       
-----------------------------------
| 5        |   (3)   |  BB       
-----------------------------------