You can import rosters into the league after the league fills up.
Once the league fills up, the import rosters option on the commissioner's league page will become available.
Until then, you can invite a few more friends to join.
If your friends are busy, you can always sign up for them using their E-mail address and a simple password, and then they can change their password when they have time to play.
You can read more about importing rosters here: