Click Memberships > Subscriptions/packs > Add new
Name it > in Payment Settings, price it at $0, then consider toggling on 'Counts as non-paid for teacher payroll. This ensure's a staff members attendance of another teacher's class will not result in the teacher being paid.
In Duration Settings > choose Absolute Interval, put 1 month
In Auto-renewal settings, toggle on 'This subscription auto-renews.
In 'what this subscription can be used for' section, choose benefits > click Create.
To add staff, click New Sale.