Open your F&F list, scroll down and click on Add a Guest. That gives you several options, so it depends on what information you have.
If you're not connected to any of them at all yet, I would click on Enter their name and age and then complete that information for the person who has the
MDE account, with the exact name they used to create their account (i.e., if they're Robert Smith, don't put Bob Smith).
Or if you know their Disney resort information, you can click on Import from a Walt Disney World Resort hotel reservation.
Or if you're already connected to one of them, say a parent or spouse, you can click on Find through my connected Guests, and it should pull up a list of their children or spouse or other people that are in their MDE F&F list, and you can click on the ones you want to connect to your MDE.
If you're using the MDE app on your phone, and you're physically close enough to the person, there's also a Scan their linking code dialog that you can use, but it requires putting your two phones next to each other!
Good luck - I saw your post on the Y&B thread - y'all are going to have an extraordinary time! Best wishes!