Buy VIP with RP

i am assuming there is a way that the user once obtaining enough RP can purchase an item that will run a command that will add them to a certain group i.e the VIP group. that said, i am having trouble figuring out exactly how i could do that. Could someone point me in the right directioin if this is possible?  (i know its possible as i've seen it on other servers just don't know if this is the correct plugin or not)

TIA - Spec


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i figured it out... 

rewards add command "lotto" "oxide.usergroup add user $ lotto" 20


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so i thouhgt i figured it out, but when i pull the UI and go to commands it just shows a "--" when it should show "lotto". also when i purchase it, it executes and my RP is taken away but im not added to the "lotto" group. anyone have any idea what im doing wrong?