Hey Rico. How are you using the web services? Are you setting them up as receive data connections in the InfoPath UI?
Once you put in the http://servername/qdabrawebservice/aduserinfo.asmx you should see all three methods listed. I would recommend you add all three seperately to a blank form and then display all of the returned data and play with them.
GetMyInfo will just return the username of the user.
GetUserInfo takes a username and returns all of the keys. You have to use the filters to get exactly what you need.
IsMemberof needs the group name and then returns true or false.