The cancelled class is the whole problem
Somebody books a 6am class, sets an alarm for 5:15, drives fifteen minutes and finds a dark room. That is the experience that loses a member, and it is almost always preventable.
A single message the night before fixes it. NORTH YOGA: no 6am Vinyasa tomorrow, Sarah is unwell. 9am and 6pm running as normal.
Naming what is still running matters as much as naming what is cancelled. A member who only reads the cancellation assumes the whole day is off.
Substitutes, room changes and the small stuff
A substitute instructor is not a cancellation, but members who come for a specific teacher want to know. Saying so honestly keeps more of them than hoping nobody notices.
Room changes, a studio closing early for maintenance, the car park being resurfaced, holiday hours over a long weekend. All the same message type: something changed and people need to know before they set off.
These are also the messages that justify the list existing. A studio that only ever texts about offers trains members to ignore it. How often to message a customer list covers that balance.
What this cannot do, and what you need instead
It cannot tell one member their class is cancelled. Every member on the list receives identical wording, so a message about one booking is not something this carries.
It does not connect to your booking system. Mindbody, Glofox, WellnessLiving and the rest already notify the people booked into a specific class, and that is the right tool for a per-booking notice.
And it sends at a fixed time each day, so a class cancelled ninety minutes before it starts needs your booking app or the phone. What happens when a message does not reach someone covers the related question of delivery.
Filling a class that is nearly empty
The reverse problem is a class with four people in it that needs eight. One message naming the specific class and time usually beats a social post, because it lands on the lock screen rather than in a feed.
Keep it to the class, the time and nothing else. Space in tonight's 6pm Reformer, first four to reply get it. A general invitation to come to more classes does not work.
Used sparingly this is effective. Used weekly it becomes the thing members mute.
Building the member list
A QR code at the front desk, where people stand still while checking in. That is the single best placement in a studio, and it beats a poster on the wall by a wide margin.
Say what they are signing up for. Cancellations and class changes only is a promise members will accept, and it is also a promise you then have to keep.
Do not upload numbers from your booking system. A number given to book a class is not agreement to receive ongoing messages, and members opting in themselves avoids the question entirely. Where to put the code so people actually scan it covers the practical side.