Interesting idea.
Aside from the already mentioned legal questions, here is how I would set it up.
Create your admin user/owner account, this is the account with the plex pass associated with it. Do not use it on your remote users/tvs, use it only for administrating your plex server and users/friends.
For each individual ‘customer/business’, YOU create a new standalone plex user account (if necessary, you may need to create unique email addresses), then your admin account can invite that plex user to your server and you can SHARE whichever specific libraries/content that is applicable for that user.
Using that generic customer account, login to each of the remote plex clients (for that particular business).
Doing this will keep your admin account completely separate from your users, and will let you manage the customers differently according to yours or their needs. Also, it allows you to easily UNSHARE if for whatever reason they are no longer a customer of yours.
You can provide this customer their own plex account credentials in case the plex clients login expires or gets reset or a new client needs added.
Or you could keep the account logins to yourself if you wanted to micromanage them.
If you have your own domain, I would setup an email alias customername-plex@yourdomain.com and point it the most appropriate real email. This way you can manage the plex accounts on behalf of your customers, or point the alias to a customers appropriate business email.
edit-
one possible drawback to this approach, at some point there is probably a maximum number of ‘friends’ you can invite to access your server, and thus would either require a 2nd plex admin account (and plex pass). Or by going through plex legal you could potentially get some kind of expanded/custom licensing.
Either way, I still think this is best approach.
if you put all of your clients on a single shared user, then you have no way to limit their individual access, nor any way to remove their access (without changing the shared user account credentials and forcing every client to re-log in).
Well I guess you could invalidate the clients on the plex web account manager (this would cause the client to have to re-auth, and if the customer knows how to log back in, won’t stop them, but still… individual business plex accounts gives the most flexibility.
another (extremely different and more complex) way, would be to put behind a static domain name (of yours) and enable insecure traffic.
This would essentially open your server to public access (outside of plex user authentication), so you would either need to accept that or use some kind of firewall or vpn access to restrict access outside of plex itself.
