Right now, post your friend a message saying “please join my challenge on the main page”.
Although all challenges are “open”, the organiser can (by pressing the ‘x’) remove a person from any team. They remain on the challenge page, and you can reassign them, unless they specifically withdraw - this saves you from accidentally deleting people out of the challenge.
In due course “by invitation” makes sense, but probably not high on the rapidly growing wish list. (Unless someone else gets keen and leaps in to to it 
)