anchor.diff:  
   ## Managing group members [/{groupid}/user]
   ### Add a user to a group [POST]
  -Puts the identified user into a group.
  +Puts the identified user into a group. 200 if it succeeds, 204 if the group 
  +ID was not valid, 400 if the groupid or userid were not specified.
  +
  +See issues [#7](https://github.com/tidepool-org/armada/issues/7) and [#8](https://github.com/tidepool-org/armada/issues/8).
   
   + Request
   
  
positive.code: + Response 200
  
          {
              id: "12a34f5"
          }
          
  + Response 204
  
  + Response 400
  
  
  
  
negative.code:         }
  
  ## User Membership [/membership/{userid}/member]
  ### Fetch the list of a groups a user is in [GET]
  Returns the groups that the given userID is a member of. If found, returns
  an object containing an array of groups. If there are no groups, the response 
  is a 204 (rather than an empty array). 
  