Gmail client for Android lacks ability to open invitations in calendar and if someone invites you to an event using Outlook you can’t accept invitation or review event details. Invitation Plugin fixes that free of charge and without advertisement. Now you simply click on the invitation file attached to the email and you see event in Google Calendar application.
Great solution if your organization uses Google Apps and Outlook.
Google Calendar application is required to use Invitation Plugin - https://play.google.com/store/apps/details?id=com.google.android.calendar.
If event is not found on your calendar you can import it using your Gmail account. Gmail automatically adds events to your calendar when it gets email with invitation. Invitation Plugin can resend ICS invitation to your Gmail email address in case you have received invitation to non-Gmail email.
Gmail should add events to your calendar automatically by default. If that does not happen please check:
http://support.google.com/calendar/bin/answer.py?hl=en&answer=83117 and search for Automatically add invitations to my calendar.
In addition to email attachments Invitation Plugin opens ics files received using instant messaging applications or when you browse files in file manager.
ICS file is storage for iCalendar data and used by most email clients like Outlook, Google Calendar, Thunderbird and others.