To find the greatest number of people they can invite and stay within their budget, we first need to subtract the cleanup fee from their budget:
$2,000 - $90 = $1,910
Now, we divide the remaining budget by the cost per person to find the maximum number of people they can invite:
$1,910 / $35 = 54.57 (approx)
Since we need to round down to the nearest whole person, the greatest number of people they can invite is 54.