EPBCS REST API: Adding users to already existing group is throwing out error

Summary:

Hi - we need to add users on the fly to an existing group. The code is also attached. we keep getting the following errors even though all parameters are defined.

Log messages : 

{"links":[{"rel":"self","href":"https://epm2-xxxxx-xxxxxx.xxxxxxxxxxxxxxxxx:443/interop/rest/security/v1/groups?filename=Test72_User&jobType=add_users_to_group&groupname=Test72_Proposal","data":{"jobType":"","groupname":"","filename":"","username":""},"action":"PUT"}],"details":"EPMCSS-20656: Failed to update user(s) in group(s). Invalid or insufficient parameters are passed with command. Please provide all required parameters with REST command. ","status":1,"items":null}

Any advise or help will be highly appreciated. Thanks a lot.