I am syncing user accounts from an authentication provider, but I want to add users to an organization manually (and not based on a rule or a user attribute). Is there a way? if so, how?
You should be able to add users as guest to the organization you want them to be able to access in (bottom left) Settings > Organization > Guest.
From docs: Every user is a member of only one Organization, but can be a guest member of multiple Organizations.
Note: This might move to Control Panel in the future.