Create your formula like the following, then group on it:
//@GroupFormula
If {Table.Customer_ID} in [1,5,7,11,15] then
"Group1"
Else If {Table.Customer_ID} in [2,77,3,6] then
"Group2"
Else If {Table.Customer_ID} in [8,10,14,55] then
"Group3"
Else
"Group4"
-dave