Game invitations appear on your Home Screen and in the Notifications area. To get to the Home Screen, simply click this link, located in the top left corner of the OGS website:
To find the notification area, first click the tiny arrowhead next to your user portrait, located in the top right corner of the OGS website:
I am not sure if this resource will be helpful to you, but I’ve put together some information for newcomers to go. If you are interested, you can find it here 
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