First let me say, congratulations on being approved for the Google grant.
There are three ways that you can reach out to the Google grants team to thank them.
Contact Form - This is the general contact form. As you can imagine, they receive a lot of email via this form but I can ensure you that they read each one.
Share Your Story - This is more for existing grantees but is useful for new grantees as well. It gives you a chance to share your story with Google and how the Google grant has impacted your organization.
Posting your 'Thank You' publicly here in the forums.