Some programs are invitation only. This just means that the company wants to restrict access to their program to a smaller set of researchers. Generally, invitation-only bounty programs work the same as public programs. The only major differences are that they are private and you have to be invited to participate in the program.
If you are selected to participate in a private bounty, you will receive an invitation 24 hours prior to the start of the program. All invitations will be sent from Bugcrowd--not from the actual company running the bounty.
Invitations are usually sent to researchers who have shown over time that they consistently report high-risk and valid vulnerabilities. As you accumulate kudos and rewards over time, you earn credibility as a reliable researcher.
To view your invitations, log in to Bugcrowd.
Go to Account > Invites.
You can choose to participate in any program that you are invited to or you can opt out of them.