As long as there is a minimum of one adult in the party, the ticket will be valid. Don't forget an adult counts as somebody age 16 or over.
You could therefore have five people age between 16 and 19, two of whom are travelling as adults, and three of whom are using their West Yorkshire under 19 photo cards to qualify as concessions.
And of course, any other iteration thereof - such as one person 16 or over and four people younger than 16 etc etc