There are two ways you can monitor additional inboxes within Lavender.
Aliases are additional email addresses associated with your account. It is another identified for your primary inbox, not a separate account.
This is more common with Outlook than gmail.
To add an alias, head to Settings > Account + Integrations > Aliases
This will track emails sent from those additional email addresses (allowing you to view analytics).
You can add additional mailboxes by clicking the (+) sign on the left side of you Dashboard:
If you are on a Pro or Teams plan (paid), you are able to monitor up to 4 additional mailboxes.