The budget is $2000.
The clean up charges is $90.
The charge per person is $36.
Let the number of person is x.
Then from the question,
[tex]\begin{gathered} 36x+90\leq2000 \\ 36x\leq1910 \\ x\leq53.05 \end{gathered}[/tex]Thus, the greatest number of person they may invite is 53.