Adding the connect button is as simple as copy pasting a html snippet and supplying your application name, logo and a callback URL as parameters. This callback URL should listen to response from the 'Connect to Office 365'. Check the documentation of more details.
When the user logs in & allows access to his group, the callback URL would receive the selected group name & the corresponding WebHook URL. Sending a connector card is as easy as posting a formatted JSON payload to the WebHook URL. You can view or edit the sample JSON payload and preview it by clicking on the 'Preview Connector Card' button.
STEP 3: Preview and post connector card
Authorize the application and click on Preview Connector Card button to preview.
We will keep you posted on the developments on this initiative though our blog. If you have any feedback share it on UserVoice. Post your questions on Stack Overflow and tag them with Office365Connectors.