To get more members for your Gang, other members have to apply for membership with you and your Gang. They just have to visit the profile page for your Gang for that and click on the button “apply for membership” to send you their request.
A little red digit will tell you, if another player has applied for membership with your Gang. You then simply open your Gang and click on the pen next to the player name to accept or decline membership. Via this pen you will also be able to change the status of a Gang-member after he or she has joined the club.
To get new members, simply write them a personal message and advertise for your Gang, so that they apply for membership with you.