Please note that Ben and Mary will each have to have a Google Account and a Google+ Profile (that complies with the Names Policy) before they will be able to participate.
If they don't currently have a Google account, they will automatically create a Gmail account when creating the Google account. If they aren't going to monitor the Gmail account, they will need to add an alternate email to the Google account. They will be able to log into the Google account, using that alternate email address.
Frequently Asked Questions about creating a Google account