Suppose you have a group of 8 male and 4 female volunteers. how many ways are there to choose 6 of these volunteers for a treatment group?

Since volunteers can be of any sexe (no precision given in your problem), then we can consider the question as  how many group of 6 we can make with 12 persons:
It's a combination of 6 chosen among 12

¹²C₆ = (12!)/(6!).(12-6!)

²C₆ = (12!)/(6!).(6!)

¹²C₆ = 924 Ways