Add user to a group using /rest/api/1.0/admin/groups/add-user not adding the user

[SVVP-2018]

I am trying to use the rest api to add user to a group.

curl -X POST -u username:token -d '{"context": "GroupName","itemName": "userName"}' -H 'Content-Type: application/json' https://bitbucketurl/rest/api/1.0/admin/groups/add-user

I am not getting error but I see the user is not getting added.

something i am missing ??