Group discounts nowadays are very little and usually at the same level (or even worse) as for individuals booking them online. Thus we decided, like usual now, that you should book your skipass online or after arrival on your own.
The valley of Gastein is part of the bigger skiing area Ski Amadé. Ski passes for 2 days or more are sold as "Ski Amadé" skipasses. Ski Amadé is a huge combination of 28 skiing areas in Salzburg, with more than 270 lifts.
Skigastein / Ski Amadé does dynamic pricing, thus booking online upfront is slightly cheaper.
Ticket selling has not started yet, expect it to start in October!
https://www.skigastein.com/en/tickets-operating-hours/winter-tickets/multi-day-tickets/