congratulate s.one with/on/upon; are these preps all correct?
less people/fewer people; are both possible?
Congratulate someone on/upon
'Fewer people' is the grammatically correct version as it's a plural countable noun. However, many native speakers use 'less people', including educated ones. It seems that 'less' is growing in popularity. I would say that it is better to play safe and use 'fewer', especially in formal language and exams.