Ask HN: Can I send geography activated texts to an opt-in cell list?

My company is in fashion and we're launching an important launch party in December. At the end I want to create a "scavenger hunt", giving text based clues at different locations in the city. For instance, I want to to detect if the people on the list are in central park, and then send them a text with a clue.

Is this possible? What apps could help accomplish this?

Thank you HN!