Add Content Editor Web Part on the page and in source editor insert the following code
<a href="">Email Us</a>
open a site page in SharePoint Designer, select the location where you have to put the link and then insert above code
if you want users to directly open email link on a page when they click on "Email Us / Contact Us" button then assign "OnClick" event to Contact Us button / tab with above code 
<INPUT TYPE="button" VALUE="Email Us" onClick="parent.location=''">