You can Use built-in if there is an extension to expanding dining room table. Many tables dining come with a foldable extension panel or a split in center where leaves can be inserted to add extra seating to people; sometimes you can give capacity for 6-8 people more this way. Follow instructions of manufacturer to expand your table.
If you expanding dining room table, or if you are still not enough space, buy gaming tables that have same approximate width and height of table in your dining room. Put them on ends of the table, and cover with same tablecloth to create an illusion of continuity. Just make sure your clients are aware that they will not be as wide as rest of table.
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If expanding dining room table is of irregular shape or if tables are too flimsy, large table with a large and sturdy board, put a cloth or other cover over table to protect it, then put board down on top and cover with an additional tablecloth. If your board requires support at both ends, you can place a stool or other object of correct height under sections of board that needs support. Again, warn your guests of array so that no move support or tablecloth error.