From the AppGallery home page:
1. Click Collections on the menu bar.
2. If necessary, click on the organization name and select another one in the list.
3. Click on the Collection from which you want to remove a user.
To Remove a User:
4. Click on the Users tab (which appears similar to the one below):
5. Click on the 3 vertical dots next to the user you wish to remove
6. On the confirmation message, click Remove.