Apps have similar governance rules to Projects - they must be explicitly shared for others to add and use them in projects.
To share an app:
Go to manage account -> apps
Click edit next to an app in the list
Scroll down in the edit modal to the sharing section.
Invite users by email, or entire workspaces.
Give users view access to use the app in projects.
Give users admin access to allow them to edit the app configuration, share the app, or delete the app from the database.