Unfortunately, Group Ring for Calling Groups was not introduced until Magix R3. In Legend R7 your Calling Group types are Circular, Linear, and Most Idle.
What you can do, is take an unused single line station port and an unused LS CO trunk port, and loop them together with a line cord. Assign the Line to the extensions that you want to ring. Call the extension number of the single line port, and the phones will ring on the Line button.
If a call is not answered, it will cover to the extension on the lowest logical ID's voicemail (if you have that coverage), so I would also assign the line to an unused Adjunct extension, and make that adjunct the Principal User of the Line. Be sure to name the Adjunct something meaningful so you can identify it years later when you don't know why it is set up that way.