ANSWER: Hillstone rarely accommodates parties over four and strictly prohibits combining tables or hosting large groups.
For parties of three or four, a tiny allocation of tables occasionally opens on their website 7 days in advance, but they disappear almost instantly. Unlike pairs, walk-ins for four are rarely accommodated since tables are tightly managed.
For groups larger than four, their policy is an outright refusal. Hillstone explicitly bans booking separate two-person tables with the intent of sitting together, and because their booths and tables are bolted to the floor, they physically cannot push tables together.