Your email address (and phone number if you add it to your profile) will be viewable by your Foursquare friends by default.  

In your account settings, you can

• choose whether or not you’d like to display your email address or phone number to your Foursquare friends and
• you can edit your email address or phone number.  

Even if you choose to let your friends see this information, it is not viewable by the public.