On the Share Office pop up, choose one of the below options.
Invite via email
Enter their email and select Invite. The email you use to invite them should also be the email they use for their shared Microsoft 365 subscription. They can then click on the Accept link in the email to Get Started installing Microsoft 365 Family.
Invite via link
Select the Copy button to send the link via email, text, or other message to the person you want to share with. When they click the link and sign in with their Microsoft account, they'll be added to your subscription.