You can respond to meeting invitation either through Google Calendar or through the email invitation. If you're using Outlook, the easiest and most consistent way to reply to meeting invites is through Google Calendar. That process is documented below. When you receive a meeting invitation in your Outlook email, follow these steps to accept or decline a meeting invitation:
NOTE:
If you choose to use the Outlook email client, meeting requests will not behave as you expect and you must follow the above process. For example, when a meeting invitation arrives in your email and you accept the invitation, the meeting will not display in your Google Calendar as being accepted. This is because calendar is in Google and your mail is in Exchange. Once the migration is fully completed, and all mail and calendar are in Google, you can accept a meeting request via your email the same as you could when everything was in Exchange.