There's a new "phishing" email going around that uses some very tricky methods to seem like a legitimate email and encourage you to sign in to your Google account to access a shared document. It is extremely difficult to detect that it isn't legitimate. If you get an email from google asking you to sign in to view something, simply go to your web browser, head to google.com, and sign in there.
World's shortest FAQ:
Q: When is it safe to click a link in an email?
A: If the link takes you to a webpage that asks you to sign in, you're at risk.
Q: Can my Mac get infected by a virus by clicking a link in an email or reading it?
A: No. Currently, the only way too get malware installed on your machine is to run an installer and then authenticate with an administrator username and password. Any time your computer asks you to enter your password, think carefully about it before you do so. It's the same as letting someone into your house—if a guy shows up and says he wants to come in to fix the stove, think to yourself, do I have a stove? Does it need repair? Did I call anyone about it?