curl -v -u admin:geoserver -X POST -H "Content-type: text/xml" -d "<User> <username>testu</username> </User>" http://localhost/geoserver/rest/usergroup/users

Note: Unnecessary use of -X or --request, POST is already inferred.
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 80 (#0)
* Server auth using Basic with user 'admin'
> POST /geoserver/rest/usergroup/users HTTP/1.1
> Host: localhost
> Authorization: Basic XXXXXXXXXXXXXX
> User-Agent: curl/7.47.0
> Accept: */*
> Content-type: text/xml
> Content-Length: 41
>
* upload completely sent off: 41 out of 41 bytes
< HTTP/1.1 405 Method Not Allowed
< Date: Tue, 24 Jan 2017 04:57:58 GMT
< Server: Noelios-Restlet-Engine/1.0..8
< Allow: GET
< Content-Length: 0
<
* Connection #0 to host localhost left intact