The old way was to set up a fleet with someone and invite people who wanted to join one by one once they had requested that you do so. Now you can post a general invitation and let people click that invitation to join the fleet, putting the work in their hands.
To set up a self-invite fleet:
1: Start a fleet. You can either choose to invite someone or you can rt-click yourself and form a fleet of one (unfortunately you still need someone else to get gang boosts, or I'd be in a fleet all the time).
2: Rt-click the little triangle on your new fleet window and set the channel to self-invite. You are given two options: corp or alliance (if your corp is in an alliance).
3: Go to the channel you want to invite from, rt-click your name or your avatar and click channel->post fleet invitation. Don't forget to tell everyone what the invite is for!
Step 3 will work no matter who you are in the fleet. Remember that new people may be entering the channel all the time, so in case of disconnects or new people you might wish to repost the invite every few minutes.