I might be missing something, but why don't you just add that user as a member of the ring group?
You can also give them a "Group Presence" for that Ring Group so they can toggle themselves as Present/Absent from the Ring Group.
I know that sometimes we don't want the phone to actually ring, just monitor the ring group for calls.
If we feel like answering the call we will by pressing the multicall key, otherwise we just let it go.
You can also set a delayed ring on the button.
I know this is all probably old school stuff left over from when a ring group didn't have many features like cascade ring etc...