If you would like to share it with someone to work with you there is an +invite button which will give you a link to a multiplayer session. If you'd like to just generally share your code simply copy the url in your browser and share where needed.

Actually, repl.it has changed, and there will now be an +share button instead of an +invite button. It will be in the upper-righthand corner. You can click on it and share it to someone. You can either invite by link, or invite by username/email.