The group chat has fourteen people and one open Saturday. Everyone saw it. Nobody owns the yes.
Ask one person.
The shift is the whole message
Saturday 14:00–22:00, your name, their name, the date you need an answer so you can ask someone else. The deadline is real or it is decoration.
One line of why, if it changes whether they can help — a flight, a kid, a class. Not a memoir.
They are probably on email
Retail, hospital, warehouse, bar. They do not need an account. The reply is the point. If your workplace has a scheduling app, the briefing is how you got the human yes; you still tap the swap there.
Do not nick them in the letter
Whatever you call them in your phone is not their name on a work message. Use the name on the roster.
If they say no, you still have time. That is what the deadline was for.