CATEGORY.AllowUserGroups = { "admin", "VIP", "Owner", "Mod", "HeadAdmin" }
Seems obvious. Lua is case sensitive though.
Make sure your groups are typed out there exactly like they are in your Gmod groups.
VIP is not the same as Vip is not the same as vip is not the same as vIp.
If that doesn't work for you, then it's beyond what I know of pointshop.
There are other pointshop discussions in this forum section that might help.
Type the word pointshop in the search box at the index page of Dev corner, it will only search dev corner.