How do I add someone's email address so it is listed when I try to address an email?

like QuickTimeKirk has already said, add them to the contacts. if they are already there, you needed to connect the contacts app to the mail app. that is something you need to do when adding your e-mail account to the mail app. when adding the account it asks you to select which apps you want to use with that account. you need to ensure contacts is selected in that step.

i've looked around to see if i could find where to add the contacts app to be used with my mail account and it seems as though you can only do that when setting up apple mail. if you didn't do it then, you will need to remove the account from the mail app and re-add it.

Add them to Contacts app.