Add cohort prices on the activity type. To make it members-only, turn off Standard Price and add prices only for the cohorts that may join. Players who aren't in one of those cohorts can't sign up.
How to set cohort prices
Go to Play → Activities → Types and open the type.
Open the Prices tab and click Edit.
Add a price for each cohort.
Click Save.
How to make it members-only
Open the type's Prices tab and click Edit.
Turn off Standard Price.
Add prices for the cohorts (for example, your members' cohort).
Click Save.
Good to know
Memberships give players cohorts, so you can price by membership through their cohort.
Future sessions take the new prices unless a session's own pricing was changed (Inherit pricing from activity type turned off).
You can also restrict discount codes to specific activity types.
