Try this:
Open population.dat in either notepad or notepad++ and look for the following section:
TRIGGER
BEGIN
TRIGGERNAME "Greasers"
AREACODE 0
POSITION 111.341003, -30.775801, -2.830120
YAWPITCHROLL 0.000000, 0.000000, 0.000000
YAW 0.000000
ZHEIGHT 20.000000
ISMISSIONSPECIFIC FALSE
ISLOCKED FALSE
AMBIENT 0, 0, 0
USETIMECYCLE FALSE
HASPOPULATIONDATA 1
FORCEPOPREAD FALSE
POPULATIONDATA
BEGIN
POPULATION_DAY
BEGIN
TOTAL 15
PREFECT 0
NERD 0
JOCK 0
DROPOUT 0
GREASER 15
PREPPY 0
STUDENT 0
COP 0
TEACHER 0
TOWNPERSON 0
SHOPKEEP 0
BULLY 0
END
POPULATION_CLASS
BEGIN
TOTAL 1
PREFECT 0
NERD 0
JOCK 0
DROPOUT 0
GREASER 1
PREPPY 0
STUDENT 0
COP 0
TEACHER 0
TOWNPERSON 0
SHOPKEEP 0
BULLY 0
END
POPULATION_NIGHT
BEGIN
TOTAL 6
PREFECT 0
NERD 0
JOCK 0
DROPOUT 0
GREASER 6
PREPPY 0
STUDENT 0
COP 0
TEACHER 0
TOWNPERSON 0
SHOPKEEP 0
BULLY 0
END
POPULATION_CURFEW
BEGIN
TOTAL 0
PREFECT 0
NERD 0
JOCK 0
DROPOUT 0
GREASER 0
PREPPY 0
STUDENT 0
COP 0
TEACHER 0
TOWNPERSON 0
SHOPKEEP 0
BULLY 0
END
END
HASAMBIENTVEHICLEDATA 0
FORCEVEHREAD FALSE
END
Change all of the GREASER population numbers and totals for each part...morning...afternoon...evening and late evening to whatever number of greasers you want. I would suggest just changing it to the exact number of greasers thats in the greasers clique and no more then that since more peds will slow your pc down unless it is high end. Try this edit and let me know if it adds any to the blue balls and if it does not I will locate the override for this and post what you need to know about that.
Sry that it took me a bit to really try to give some help with this but I have been very bogged down lately and feeling rather like crap.