Find a group of clients
Open Menu, then CLIENTS & BOOKINGS, then Clients.
Quick segments are the one-tap presets. For anything more specific, the filters are Last visit, Lifetime spent, Service had, Lifecycle stage, Tags, and Owes money, and they stack.
If you end up with nothing, the screen suggests the fix rather than just shrugging: try widening the last-visit window, lowering the spend tier, or clearing the service filter. Clear filters resets everything.
On a team you can switch between Your book and Team clients.
Once you have the group you want, Message these clients sends to all of them. You can also tick individual people and use Message selected.