You can include guest passes to a client's membership/subscription.
Note: guest passes can only be included in memberships/subscriptions!
Adding a guest pass to a subscription/membership
Go to Set up > Pricing Options
Create or edit current membership
Under "Guest Passes" toggle on "Include guest passes each billing cycle" and enter the number of guest passes that are included each billing cycle.
This will grant clients guest passes each billing cycle. Guest credits will be added as a separate pricing option on the client's account with the total amount of guest passes they are allowed to use and identical access settings to this pricing option.
Video demo