There’s an existing feature suggestion discussing an option to give friends an invitation link vs. inviting them “manually” (e.g. giving them the opportunity to use their contact already linked to a Plex account vs. an e-mail address they’ve shared with you). I suggest you comment/vote in that thread in order to avoid distracting or cannibalizing votes.
I’ll therefore close this thread as a duplicate.
Keep in mind – Plex isn’t primarily made to publish a link to give access to your media in public places. That’s getting them pretty close to file sharing and the idea is generally frowned upon. Plex have already rejected a number of suggestions aiming for such use cases. The common understanding is you’re sharing access to your library with family and friends whom you know or with whom you can easily align when you want to invite them (e.g. to get the right contact details) – not a public board à la “want movies, click here!”
As a side note: don’t hand out your credentials… with those, anybody can take over your server and cause havoc.
