You would need two cabs, they are small and only hold 4 people. With that many people I would consider either taxis or a small tour, or maybe look into renting a van for the day and drive yourself. It is an easy drive, straight shot down Highway 307 and watch for the signs for "Tulum Ruinas" before you get to Tulum pueblo. Maybe hit the beaches on the south end of the hotel zone after your tour. Bring pesos, the ruins only accept pesos, not USD or credit cards.
Thanks everyone for your very helpful advice! We are headed off today.
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