You should have a wrapper in there that can be customized to include your group's name and website. Just go to:
"Start organizing" --> "Email Wrappers" --> "Basic Custom Wrapper"
Under where it says "Email wrapper foot HTML," you'll want to replace "Your Group Name" with, well, your group name. To add a link, look for this code:
<a href="#" style="text-decoration:none;color:inherit;">Sample Link</a>
Replace the # with your full link -- e.g.
350.org/. Replace "Sample Link" with whatever text you want to hyperlink -- often this is just your written out URL (e.g.
350.org) but can also be something like "Click here to learn more about our work."