Admin-level users can add new users. If you are logged in with an admin-level account, you can add user accounts to Nextcloud as follows:
Click on the icon in the top right corner of the home screen and select "Users".
This will open the user management view. Click on "New user" in the top left corner.
A window will appear in the center of the screen where you can enter the new user's information, such as username, display name, password, email address, and any groups to which the user should have access. As the second-to-last option, you can also assign the user as a group administrator. Lastly, you can set a quota for the user's storage space. Once the information is entered, click "Add a new user".
Done! The added user will appear as a new row in the user management view. You can further edit the user's information by clicking on the pencil icon at the end of the user's row.
For more instructions, you can refer to the official Nextcloud documentation available at: https://docs.nextcloud.com/server/stable/user_manual/en/index.html