How can I add a Participant List to a new SignUp without sending invitations?

There are a few ways a person can be added to the Participant List of a SignUp:

1. Direct invitation sent via SignUp
2. Entering the SignUp by a Shareable Invitation link (even if they don't sign up)
3. An Organizer gives the Participant an assignment manually

While it's NOT possible to add people to the Participant List outside of the ways above, you CAN keep a copy of the Participant list, stored in a Contact List, and add new people there without inviting them. Please see: How Do I Import Emails or Create Contact Lists?

Once you've created a Contact List, you may:

• Send a message to people on the Contact List using Advanced Messaging (in the My Tools tab). Learn more here
• Invite people to sign up from the Contact List. Here's how

NOTE: The Contact List is owned by the SignUp account holder and is NOT transferable. 

See also: What's the Difference Between a Contact List and SignUp Participants?