I read the blog post from 14 November - “Monzo With Friends - New iOS Update” and at one point is says “Once you’ve enabled payments in the new Contacts tab, you can instantly see which of your friends already use Monzo”.
I’m probably being dumb, but I can’t see anything in the Contacts tab that would let me “enable” anything - it’s just a long list of my contacts with an “Invite” button against each one, so I’m not sure what to do next.
Select someone to make a payment, doesn’t matter who as you can cancel before completing. If you haven’t already done so it will ask you to enable payments. At least that’s how I remember it, I can’t check now that’s it’s enabled.