Slight Update To Jenkler's approach, not sure in which version this changed* but:
modify the key "value" with the array of users you want to give admin permissions to
"value" : [ "1", "2" ]

as opposed to
"members" : ["1","2"]

Note: the ObjectId() used in the first parameter of the update query is the id of the object found from the find query { "_key" : "group:administrators:members" }
If you have rockmongo you may also just edit the result that you find from { "_key" : "group:administrators:members" }