The invitation link (a.k.a. invite) is a unique URL for True Balance page in Google Play, which is generated only for your account. You can find the invites from the message you share with your friends directly in True Balance app.
Or you can find your invitation link from:
1) Drawer via ≡ > Settings > My Invitation Link,
2) Earn > Scroll down to the bottom of the page.
Please kindly share your invite if your friend is asking.