Access decides who can join your workspace and how they sign in. Add an email domain, like yourcharity.org, and anyone with an address there can sign up and join automatically. Turn on Require Google login to allow Google sign-in only, with no email and password. Find it at Settings → Access. To change what one person can do, set their access level under Members & guests.
Settings, Access: email domains that may join automatically, and an option to require Google login