If you bought a camera
Check the buy.matterport.com email address from where you bought the camera. Invites are sent there after your purchase.
Check your Updates or Spam folder in case the email was categorized incorrectly.
You can also open my.matterport.com and click "Trouble Logging In?" and enter your email address reset your password.
If you are a Collaborator
If you were invited to join Matterport but you lost the email invite, first go to my.matterport.com. Click on "Trouble Logging In?" and enter your email address to reset your password.
If this doesn't work, then ask the admin to resend the invitation. The admin should open Matterport Cloud and go to the Pending Collaborators page. Click on the three dots next to the person's name and resend the invite.
You can also check the deleted users page and reactivate a collaborator.