How do I send an email to a tag or audience from the TO line?

From the Cloze People section, you can search for a specific tag or filter by an audience, select the contacts, and then send a regular email or Mail Merge email. 

You can also simply, enter in the hashtag symbol (#) and start typing the tag or audience name. 

1.) Open the email composer and type the tag or audience name. When it appears, tap on it to select it.