Click ‘Grant Library Access’ in the upper right, and then punch in their email address. When it appears (or their user appears if they happen to already have an account) choose them and go through the prompts.
If they don’t already have an account, they will get an onboarding email to sign up for Plex, which takes care of that step for you.
Big key is after they go through the process, you have to walk them through accessing your server with the libraries you shared with them, since without guidance they will likely NEVER see your server content, and only think Plex is a free streaming service for the content Plex offers, because of the way they have designed the onboarding experience.