Board Thread:Questions and Answers/@comment-108.252.169.165-20151129165513/@comment-71.210.15.101-20151202024207

It is probably because they have not accepted your invitation. They will receive an email with a link. Clicking on the link will take them into the wiki, where they need to click the green accept box. You'll need to close/open the game after they accept.

Once done, they'll show up on the bottom of your Portal page, and will show in the Gifts. Once they've played long enough to see Contracts, you'll also be able to send trains to them. (If I recall, Contracts show up after level 10.)