addUserToGroup fails on Ruby

#<SOAP::Mapping::Object:0x111356c>: com.atlassian.jira.rpc.exception.RemoteValidationException: group cannot be updated, group name cannot be null, needs a value (SOAP::FaultError)

This error is thrown even tough the group used as parameter is valid.

Please see attached code for more details. In order to make it work you will need to replace the following information:

DefaultEndpointUrl value on defaultDriver.rb
Username and password for login on user.rb
Group name for getGroup on user.rb