https://support.plex.tv/articles/categories/features/remote-access-server-sharing/
The user adds him/herself, by creating a plex.tv user account for themselves, over on https://plex.tv (“Sign Up”)
Once this user account has been successfully created and the email address that is connected with the new account has been verified, you can invite the user to share your media.
If the user is a member of your immediate family, you can also add him/her as a “managed user”. But doing so will tie your user accounts together much closer than you might be comfortable with. So inform your self first about the implications before doing so. https://support.plex.tv/articles/categories/features/plex-home/
You don’t need to. After enabling remote access to your server, any Plex app which is logged into a plex.tv account with access permission to your server will find your server automatically.
As explained above. The most basic case is to open your web browser and go to https://plex.tv
Sign into your account, then open the hosted web app.
Provided, that remote access to your server is working as intended, you should see your server immediately.
Start by enabling remote access. The most important thing for that is your internet connection. Some (often newer) contracts don’t provide you with a publicly routable IPv4 address for your router. But this is very important for remote access.
Go through the first 2 articles in the first link above.
Additionally read The Basics of Remote Access Troubleshooting if it doesn’t work.
If that still doesn’t help, ask your ISP whether they have put your connection behind a CG-NAT. And if they did, under which circumstances they are willing to disable CG-NAT.
