This works for me
/webservice/rest/server.php?wstoken=xxx&wsfunction=core_user_get_users_by_field&field=id&values[0]=2
2 is the user id.
You can use any field that uniquely identifies the user. eg: field=username
You can also retrieve more than one user at a time eg: values[0]=2&values[1]=3
This is assuming the function was added as a web service following these instructions