Guide
Run a fest or an event team from one chat room
Organising anything with volunteers means a group that changes weekly, includes people you have never met, and has to be joinable by somebody who turned up on the day.
Volunteers should not cost you a contact list
The usual approach is a group where somebody collects forty numbers and adds people one at a time, and every volunteer ends up holding forty strangers' numbers. For a fest committee that is both a chore and a privacy problem nobody consented to.
A link solves both. Put it in the volunteer form's confirmation, on the coordinator's poster, in the department group. Whoever needs to be in the room opens it.
One room per team, not one enormous one
Rooms are cheap to make, so make several: logistics, sponsorship, stage, hospitality. A volunteer joins the one they are actually in, and cross-team coordination happens between the coordinators rather than in front of two hundred people.
Direct messages exist for the conversations that should not be in a team room at all — a budget question, a problem with somebody's shift.
Keeping order on the day
Pin the run sheet so it stays at the top. Mute what you are not running. Mentions reach the person named, which is the difference between 'someone please handle the mics' and it being handled.
Admins can time somebody out, remove them, or ban them from rejoining, and can block words the room does not want. Rooms open to anyone with a link need that, and it is there without a paid plan.
Afterwards
The room stops being used and that is the end of it, unless the owner deletes it or the team keeps it for next year. Nothing is published to the web unless an admin explicitly publishes it.
Questions people ask
Can volunteers join without an account?
Yes. They open the link, pick a name, and are in.
Can I stop people joining once the team is set?
Make the room private, or stop circulating the link. Admins can also remove or ban somebody who should not be there.
Is there a limit on how many rooms I can make?
No. Make one per team.
Or look at what is already being talked about: public chat rooms, events chat rooms, community chat rooms.