Consider switching out your dining table for a round or square one, which will let you put more people at each side than a traditional rectangular table.
Remove chairs from the table and replace with benches. Benches take up less room than bulky chairs and more people can slide onto a bench.
Place a small folding table at one corner of the room when you have extra company for lunch. Cover with a tablecloth if you want it to blend in.
Remove any furniture in the dining area that doesn't allow for seating. Things like buffet tables and china cabinets can go so there's room to sit.
Set food out in the kitchen and let people make their plates there buffet style. This will ensure the whole table is free of food and can fit more people seated around it.