Track opens when sending to groups
Tracking individual opens within groups is a feature available at the Starter tier and above.
Emails that you sent to multiple recipients before you upgraded to the Small Business tier can't be tracked down to the specific person who opens them. But all the emails you've sent since you upgraded can be tracked.
If you sent an email to a mailing list (for example, email@example.com), we can't tell which person opened the email - the tracking will say Someone.