Our SignUp Others feature (Can participants share an email address or sign up others?) allows participants to signup themselves and/or others using one email address for contact.
This feature is only in the Participant signup flow, not in the Organizer Assign Spots flow. As an organizer, you can assign their spots with multiple people connected through one email address, using this process:
1) Make sure you are Signed Out of your Organizer account on SignUp.com.
2) Use the Shareable Invitation link, just as participants do to access the SignUp.
3) See the instructions in this article about how a Participant can Sign Up Others:
Can participants share an email address or sign up others?
Simply put, you will access the SignUp and sign up the multiple participants, just as they would sign up themselves.