When you send a tracked email to a Google Group, Mailtrack is unable to identify or track the individual members of that group. Because Google Groups act as a single alias that forwards the message to its members, Mailtrack only sees the group address itself and not the list of people who actually received the email. As a result, your tracking report will show the email as sent to a single recipient, even though it may have reached many more people through the group.
This also means that any opens shown on the report reflect activity detected on that single tracked message, not verified individual opens from each group member, so they should not be treated as unique per-recipient opens.
How to get individual tracking data instead
If you need accurate, per-recipient tracking data, send your email as a campaign with each recipient added individually (for example by importing a list), rather than sending to a Google Group address. This allows Mailtrack to generate an individual tracked copy for each recipient and report opens and clicks on a per-person basis.