A new update to Google Calendar is rolling out on the web that brings the ability to add a note to a calendar invite as well as propose a new time. The new feature is rolling out to all G Suite accounts, including the free ones that most consumers use, over the course of the next few days.
Once you have the update, you can open up a meeting that you have been invited to and at the bottom of the meeting invite, can propose a new time or add a note, both of which will be sent to the meeting organizer.
The feature works not only across G Suite domains but also works across Microsoft Exchange accounts too. That means you can send proposed new times from either Google Calendar or Exchange and it will be received.
For now, the feature is only available on the Google Calendar for web site but it will be rolling out to the Android app starting on August 13th. For now, there is nothing that you need to do. The new feature is a cloud-side push to your Google account so you will know you have it when you see the ability to propose a new time. Also keep in mind that you will only be able to see the feature on events you are invited to, not those that you create.