Attaching documents with Google Drive
Trello will let you attach documents from a card directly from Google Drive (formerly known as Google Docs). Just click "Attachment" and choose "Google Drive" from the back of a card. Files that are attached from Google Drive will open in Google Drive in a new window, can be edited and saved and will not have to be reattached to the Trello card.
Google Drive is Using the Wrong Google Account
When you go to attach a document from Google Drive, Trello loads a form that lets you pick which document you want to attach. This form is provided by Google and will use whichever account you would get when you visit https://drive.google.com. When people have multiple Google accounts, sometimes the form that loads in Trello is the wrong account.
As a workaround, you should be able to go to https://drive.google.com and switch accounts. If that doesn't work, you may need to log out of all of your Google accounts and then log in to only the Google account which you need to access.
Google is supposed to be offering a "switch accounts" feature directly in the form, but it hasn't happened yet. We're also considering letting you choose which Google Drive account you want to associate with Trello, but that's not available at this time.
Google Drive is Greyed Out
If you are unable to attach a Google Drive file to a Trello card because the Google Drive options is greyed out, please try disabling and reenabling any extensions that you have running in your browser. Some browser extensions (especially ad blocking/anonymous extensions) have been known to block Google Drive, but disabling and then reenabling them should resolve this issue.