OK, some feedback…
I managed to find the outboundSMTP.properties file and the inboundSMTP.properties file.
Modified the various entries to point to my ISP's mail and smtp servers, included a valid userID, password, etc.
Now when I do an invite as any site manager, the invite has success and a few mins later there's an invitation email waiting. Clicking on this email of course opens to Alfresco Share to login as that user and the invited user is a site colleague.
Still have the problem of a fail message when clicking "Accept" or "Reject" on the task list…