A question on Friends option

@mccltd said:
If I add one as a friend I see it sends them an email. From this email do they setup their own Plex account and it logs them into my server with their own defined email address and password?

Yes.
If they already have a plex account, you can use their existing Plex user name instead.

Just wondering, as if that the case then I can see they can easily add other devices under their name.

Yes.
You must make it clear to them how many parallel video streams you are allowing.
You can monitor usage with PlexPy
If they don’t obey, stop sharing with them.

Also any other info about the differences between My home and friends?

Fast User switching is not available for friends. (that’s a good thing)

Also, Friends don’t inherit your Plex Pass perks. If they want to use the ‘mobile’ Plex apps on iOS and Android, they’ll have to pay for them (either with a one-off payment or by getting a Plex Pass subscription of their own).

If they don’t have a Plex Pass of their own, bandwidth of a Relay connection is limited to 1mbps (with PP it is 2mbps).

https://support.plex.tv/hc/en-us/sections/200295083-Server-Sharing