We would like to share out private snap package (published on Ubuntu store) with a limited number of users. Is there any API to add collaborators to snap package automatically and without email confirmation? Thank you!
Currently the only way to add collaborators for your snap package is indeed by adding the emails of the people with whom you want to collaborate (so that the store can verify that they have an account, or ensure they create one).
But note that although this will allow those people to install the private snap, a collaborator is intended to be someone who collaborates with the development of your snap - they will also have access to upload and change the snap (ie. it's not read but write access).