I was interested in this as I have a season ticket with my dad, who is getting on and can't make every game.
Previously if he wasn't going I could just ring up and upgrade the ticket from over 65 to full adult (it was briefly possible to do this online, but that didn't last long).
I contacted supporter services on Twitter and asked them the following:
Under the mobile ticket system, what happens if I want someone to use my ticket for a particular game and they are not a Citizen member?
Hi Ian, The person you are forwarding the ticket on to needs to meet the criteria in place for the actual game. For example, if you want to forward over Everton, Liverpool or Man Utd they need to be a Cityzen Matchday member and have attended three previous PL games. If it is for a game that is on general sale, and doesn't have a sales criteria attached, you can just add the person to your friends and family and you can forward it over. Hope this helps!
This is an automated response as when I emphasised the point about them not being a Matchday member, they just sent the same response.
It does seem that you can register and get a supporter number without paying for membership though using the process above.