"सब हँस रहे हैं।"
Translation:Everyone is laughing.
I suppose 'all of them' refers to every person within some specific group denoted by 'them' while 'everyone' refers to every person without riders. You can translate 'All of them are laughing' as 'वे सब हॅंस रहे हैं'.
Eg: क्लास की लड़कियों को मेरा चुटकुला अच्छा लगा। वे सब हॅंस रही हैं। - The girls in the class liked my joke. All of them (ie, all the girls) are laughing.
मेरा चुटकुला बहुत अच्छा था। सब हँस रहे हैं। - My joke was very good. Everyone is laughing.
OK.. so I guess English is not your first language? In English, "everyone IS laughing" .. it's just one of those things about English. Every "one" ... so "one is laughing". It is treated as singular even though it is plural (I know that sounds weird). So... if there are two people, then "Two people ARE laughing". "Three people ARE laughing" and so on..... "They ARE laughing". Hope this makes sense and I understood your question properly?? ()