In LD 81
REQ LST
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FEAT rnp
RNPG (leave blank)
FEAT (leave blank second time)
This will list out all phones using Pick up right now In the 3rd column you will see all the groups. Select a number that is not in use. If none are in use, start with something like 001 and go up for each group you add.
On each phone you want to put in the group, just change or add to the RNPG prompt, the number you chose