Would you at least use coasters?
The man not sitting on the table is asking the three people on the table to use coasters. It's unusual to put chairs on a table like this during a meeting, and the man likely didn't join them because of how weird it is. While he's weirdly accepted that they will sit on the table, he's asking them to at least not scratch the surface with their chairs by using coasters. It's funny because it's unclear why the people wanted to be on the table.