Whether you're on mobile or desktop, there are a couple ways you can send a message to everyone on a call.
Before joining the call:
When the meeting begins and the answer screen pops up, you have the option to Message All.
This will send a group message to everyone who is invited to the meeting.
Keep in mind that on a one on one call, it will simply be an option to message.
After joining the call:
Once the call has begun, you have the option to message all users who were invited to the meeting, or all users who are actually present on the meeting at the time.