Unfortunately, Spidergap can't support multiple people sharing the same email, though there is a workaround.
Imagine you have 2 feedback providers who do not have their own email addresses: Jim and John.
You want their invitations to be sent to their manager's email address: firstname.lastname@example.org
Send the invitations. Some email systems will accept these 2 emails and direct them both to email@example.com's inbox. If so, then you now have a workaround and do not need to continue to step 3.
If the emails do not arrive, then you will need to copy the links that John and Jim need to use and then provide these to Mike outside of Spidergap (e.g. in an email you send using Gmail/Outlook). We have instructions on how to find these links. Unfortunately, we do not currently have a more automated solution than this, but we're hoping to improve this in future - contact our support team if you have any questions.