You can group columns for output formatting only, but you can't then refer to them like you can with row groups (@I..II etc).

What you can do though is give columns names and refer to them:

| $ |    |    |    | x=2   |
| ! | p1 |    | pn |       |
|   | P1 | P2 | P3 | Total |
|---+----+----+----+-------|
|   | 1  |  2 | 3  | 12    |
|---+----+----+----+-------|
|   |    |    |    |       |
#+TBLFM: @4$5=vsum($p1..$pn)*$x

If you want to add a p4 column by putting the point in Total and pressing M-S-right you will have to manually move the pn label from the p3 to the p4 column, so it is not quite as convenient as the @I syntax for rows.

Also, you can store your constants in the table using a row marked $.

You can group columns for output formatting only, but you can't then refer to them like you can with row groups (@I..II etc).

What you can do though is give columns names and refer to them:

| $ |    |    |    | x=2   |
| ! | p1 |    | pn |       |
|   | P1 | P2 | P3 | Total |
|---+----+----+----+-------|
|   | 1  |  2 | 3  | 12    |
#+TBLFM: @4$5=vsum($p1..$pn)*$x

If you want to add a p4 column by putting the point in Total and pressing M-S-right you will have to manually move the pn label from the p3 to the p4 column, so it is not quite as convenient as the @I syntax for rows.

Also, you can store your constants in the table using a row marked $.