By default MassMailer adds an unsubscribe link at the bottom of the email. If you are looking to custom position the unsubscribe link in your emails, you need to follow the instructions below.
Log into your SendGrid account by entering the user name and the password given to you.
Click on Settings -> Tracking - > Subscription Tracking.
Click on Edit link on the right hand side to edit the "Replacement Tag".
Enter "[unsubscribe]" in the replacement tag field and click on the tick mark icon on the right side to save the changes.
Now in your Email Templates in Salesforce, you can add the replacement tag, "[unsubscribe]" in your HTML code in any position just by adding it as the href attribute value for the anchor tag.
<a href="[unsubscribe]">click here to unsubscribe</a>