I put a new account i made into a league that had advertised on here... I join and now 1/16 cars are acyually active.. Its no fun not having a challenge! But its also no fun losing 4mil to leave an onactive league
If you made the league, you can boot the inactive and advertise again. There appear to be hundreds of people downloading the app everyday, so I'm sure it will fill up again quickly.
Or you can wait till the end if the season, and leave or disband the league (you used to be able to disband in the version before the app came out and I haven't run a league in over 2 yrs so I am not the definitive answer on that part)