The sharing of this service is by invitation. No sharing of credentials is necessary.

If you have purchased Office 365 Family, you can share the benefits of your subscription with up to five other people in your household at no extra cost.

The method :

For the owner: 

Log in to your Microsoft account. Go to the Sharing page. On the Sharing page, click on Start Sharing.

Invite your co-subscribers by entering their email address or by copying the link to send it through the channel of your choice (SMS, WhatsApp, Messenger...).

For the co-subscriber : 

Open the received mail.

Click on join and connect your Microsoft account or create one if necessary. Follow the instructions.

You're ready to use the Office 365 suite!