The greatest number of people that they may invite is 78.
Explanation
Suppose, the number of people that they may invite is

The reception hall charges $ 37 per person, so the cost for
number of people
dollar. It also charges a $ 100 cleanup fee.
Given that, the maximum budget for their reception is $3000. So, the equation will be......

As the number of people will be as a whole number, so the greatest number of people that they may invite and still stay within their budget is 78.