Salseforce Web Services API does not allow API support for Professional Edition(PE) and Group Edition(GE).
You can request a special ClientID allowing your composite app to integrate with PE if you pass the AppExchange Security Review.
With this ClientID it is possible to access WS API for PE.
Following data table summarizes the API access.
|Edition||WS API (admin user)||WS API with ClientID (admin user)|
Exceptions are shown in RED color.
Also note that all other editions(Enterprise Edition, Unlimited Edition, Force.com Free Edition Force.com, Enterprise Edition, Force.com Unlimited Edition) are supporting API even without the ClientID.
Have a nice day!