You'd have to call the restaurant. Typically, it's first come, first serve for food and tables. They dont take reservations.
15 people will be a bit of a stretch. The inside is small and can be hard to accommodate. The outside maybe be possible if you join tables or sit at adjacent tables. As Derek said, they don't take reservations, so go early rather than after 8pm to avoid the rush. 6.00 to 6.30pm is usually a good time
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