Open Group → Polls, ask a question and add the choices. You can allow one answer or several. Starting a poll needs Plus, Together or an active Plus trial.
People in the group can vote and change their answers while the poll is open. When you have an answer, the organiser can close the poll and optionally mark the choice you settled on. A closed poll cannot be reopened.
For choosing when to meet, use a date poll in Plans instead. An ordinary question poll does not turn into an event when it closes.