Post what needs covering. Volunteers claim the shifts they can work, and everyone sees what is still open — on a phone, without an account for the people you are only asking once.
Sign inSay a role needs two people on Sunday at nine. The open slots are the ask, and they close themselves as volunteers claim them.
Potlucks, supply lists, field trips. Share one address; guests sign up with a name and an email, no account required.
Every org gets a public page at a name you choose — short enough for a bulletin, and it keeps working when the schedule changes.