大家好 is used to start a speech.or introduce urself etc. 你们好 means u are meeting more than one people and u want to say hello to them.

大家 everyone, 你们 you(plural) Basically it means the same thing in greeting, just like the difference between hello and hi? ↖(^ω^)↗

大家好 dajia hao ,, dajia means everyone , "Hello, everyone" 你们好 nimen hao nimen is plural YOU , "Hello you guys " In fact it is just 2 different ways to convey the same message , a greeting !

It's pretty much the same.