You can invite someone else to your learntomod world on our minecraft server (your minecraft launcher, in other words) not on the LearnToMod website per se.
If you want to use winterfell and haven't already downloaded it, press the the green/yellow 'put on server' button on the winterfell page then the next time you login, you'll be in Winterfell.
They'll login to minecraft following the same instructions you do to get to your learntomod minecraft world. You just press the start server button, get an ip and tell your friend the ip and tell them to choose multiplayer and direct connect. Remember, the ips last about 10-15 minutes so after 15 minutes, press start server again and get a new ip, being sure to share the new ip with your friend.