You gotta offer something to players that they can't get from their normal casual pool nights. If the bars offer some incentive, and the league offers entertaining functions such as banquets & tourneys & parties, then people will play. You want people to join, even if they are not avid players. The idea is to create a larger & more competitive pool community. To get people other than established players, you'll have to entice them. Make it fun & people will join.
Make the league night a Monday or Tuesday, early in the week when bars have minimum business. Require the bars to offer a free drink to each player & supply food such as pizza or cold cuts. It's a drop in the bucket compared to what the bar will be making that night, a night that usually is dead. Schedule holiday parties & tournaments, where you give away door prizes like low end cues & accessories. Have periodic league gatherings where every player can vote on rule changes & such. Make it fun and people will join. Keep it fun & it'll grow. It's pretty simple stuff. Most of all, keep it politic free. Be a dictator that listens to what the players want, but don't let anybody have enough power to screw things up.
Contact the BCA & get some ideas from them, as well as instructions for starting the league. I know some of the guys at the BCA. I played in Mark Griffin's pool room and played league with some of the BCA employees. They all know what it takes to promote the game & get leagues going. All are very nice people & will be eager to help you. We are all after the same goal, promoting the game. You getting a league going is a small step in helping the game as a whole, so it's in the BCA's interest to give as much help & advice as possible.