Hi Eckart,
You cannot use the standard workflow designer to do this. If you are using crm online, then the plug-in approach is currently your only option.
If you are using on-premise or partner hosted, a custom workflow activity would be the nicest approach as you could embed the 'join user to team' within your workflow logic.
Hope that helps,
Rob
Microsoft Certified Technology Specialist (CRM) GAP Consulting Ltd.