Do I have to accept or reject applications?

Manual approval is not required.

The app is designed to minimize friction from the sign-up to promotion.

You can delete any ambassadors you don't want in the ambassador tab.

Some merchants decide not to share their portal link publicly and only send approved leads to their sign up page. 

To require approval, you can follow the instructions in this article:

Requiring approval can greatly reduce the conversion rate for ambassadors.