If it is a small group of people you know and trust what I would do is create a custom role under Adminsitration > Role Administration
Then I would add each of those users to this role.
Then I would click the settings link on the Image Gallery and on the Security tab I would check the custom role so it has edit permissions.
Then anyone of those folks who login will be able to upload images into the Gallery, they will see links that allow them to edit and upload since they are in the allowed edit role.
You "could" use the "Authenticated Users" role but only if you are not allowing other people to register on the site. ie you could create the user accounts for the trusted friends then uncheck the box in Site Settings for Allow New Users to Register.
But I think it would be cleaner to have a custom role because later you may want to allow people to register on the site without granting them any permissions to edit anything.
Hope that helps,