Plan a new event
Pick some days that might work, share the link with your friends, and see which days everyone can do.
Tap the days that might work.
Guests won't be able to add other days. You can change this later from your manage view.
Event created
Share the join link with your friends. The manage link below is your private control panel for this event. Open it to lock in the final date, edit the details, and see who's responded.
Open your manage page now, then bookmark it. That's where you actually run this event (lock in the date, edit details, manage responses). It's also your only way back to these controls if you switch phones or browsers, so keep it somewhere safe.