The License Activation screen is located in your WP Admin at Settings > AffiliateWP Invites. And just to let you know, the license key doesn’t have any effect on your ability to use the plugin, it grants you access to support and updates.
And when you say that it’s not working, what do you mean exactly?
I’ve had a look at Invites page in Settings and I cannot see a field for entering the WP Invites licence key. I wouldn’t want to miss out on any updates.
In addition, I have set the max number of invite emails sent at one to 10. I have actually sent out significantly more than 10 invitation, however, the list of invites at the bottom (see attachment) shows only the first 10. It does not appear to index to additional pages showing all the other invites.