In SOAPUI 5.0 or higher, you can set it on the basicHttpBinding of your service from the Navigation window on the left (I think it’s the interface) (it’s the parent of all your methods and direct child of the project).
Right click on the interface name and choose “Show Interface Viewer”. Then go to the “Service Endpoints” tab. Put your authentication info there. Now you don’t have to specify authentication for every method in your service.
After entering the credentials, you need to assign to your requests. (Button “Assign” below the tabs)