Like session-based activities, drop-in activities are managed from the Activities > Registration screen.
You have 3 choices:
COMMUNICATING WITH CLIENTS
To communicate with all the clients registered for a drop-in activity, regardless of the date they signed up for, use the Operations menu in Activities > Registration and proceed per usual.
To communicate with participants registered for one or more specific dates:
Registration cancellations for drop-in activities are performed in exactly the same way as regular session-based registrations.