This piece of code does what I think you want.
It automatically increases the greatest existing group number by one. It also checks which records are already transferred, and transfers only new records.

[vba]
Sub Transfer
Dim S As String, NextGrp As String, MGIx As Long
Dim Rst As DAO.Recordset, rstEmp As DAO.Recordset, rstGrp As DAO.Recordset

'Determining next Group ID
'This algorithm works as long as the greatest Group index is smaller than 999
S = "SELECT Max(Emp_Group_No) AS MG FROM Emp_Grp"
Set Rst = CurrentDb.OpenRecordset(S)
NextGrp = "Grp" & Mid(CStr(Val(Mid(Rst!MG, 4)) + 1001), 2)
Rst.Close

'Deternmining new records in table Employee, updating table Emp_Grp
S = "SELECT Emp_ID FROM Employee WHERE Emp_ID NOT IN (SELECT Emp_ID FROM Emp_Grp);"
Set rstEmp = CurrentDb.OpenRecordset(S)

Set rstGrp = CurrentDb.OpenRecordset("SELECT * FROM Emp_Grp;")
While Not rstEmp.EOF
rstGrp.AddNew
rstGrp!Emp_Group_No = NextGrp
rstGrp!Emp_Id = rstEmp!Emp_Id
rstGrp.Update
rstEmp.MoveNext
Wend
rstEmp.Close
rstGrp.Close
End Sub[/vba]