Family Sharing Credentials not being honoured

The symptoms usually mean the affected mailboxes are not actually getting the Microsoft 365 Family subscription benefits, even though they are in the family group, or Outlook is enforcing the free 5 GB storage limit instead of the 100 GB mailbox / 1 TB OneDrive quota.

Work through these checks in order:

1. Confirm the subscription type and sharing status
   1. Sign in as the subscription holder to the Microsoft account dashboard → Services & subscriptions.
   2. Verify the plan is Microsoft 365 Family or Microsoft 365 Premium and that it is active. Only these can be shared; Microsoft 365 Basic cannot add Office apps or larger mailbox quotas.
   3. Go to the Sharing page and check that each affected email address is listed as a shared member, not just in the family group.
   4. If any account is missing, re-share the subscription using the steps in Share your Microsoft 365 Family or Premium subscription.
2. Make sure each invitee actually accepted the share
   1. For each family member, confirm they received and used the invitation email or share link.
   2. They must click Accept / Get Started and sign in with the same Microsoft account email that is used in Outlook.
   3. In some cases, they must also select Join now to join the subscription owner’s Microsoft family group; only then will the subscription dashboard show them as active.
3. Check for conflicting subscriptions or family groups
   1. If a user is already part of another shared Microsoft 365 Family or Premium subscription, they cannot receive benefits from a second one.
   2. If they see an error like “It looks like you're already in a family group”, they must leave the other family group before joining the subscription owner’s group.
4. Verify storage quotas vs. “account full” messages
   1. For each affected account, sign in to the Microsoft account dashboard and open the Microsoft storage area.
   2. Confirm that:
      • Outlook.com mailbox shows 100 GB quota (for Family/Premium subscribers), not 5 GB.
      • OneDrive shows 1 TB quota per user.
   3. If OneDrive still shows 5 GB after the invitation was accepted, wait up to 2 hours, then go to Manage storage, refresh the page, and re-check. The quota update can be delayed.
5. Distinguish mailbox vs. OneDrive over‑quota
   • Outlook “storage full” messages can be triggered by:
     • Outlook.com mailbox over its quota, or
     • OneDrive over its quota (attachments and cloud files) even if the mailbox itself looks fine.
   • In the storage page, check both Email and OneDrive usage. If OneDrive is over 5 GB and the quota has not updated to 1 TB, it means the subscription benefits are not applied yet for that account.
6. Re-share if a member lost benefits
   • If the subscription expired and was renewed, or the owner left/recreated the family group, some members may lose access.
   • Remove the affected member from sharing, then re-send a new share invitation and have them accept again.

If, after these steps, a specific account still shows only 5 GB/limited storage while others in the same Family subscription show 100 GB mailbox and 1 TB OneDrive, the issue is almost always that the email used in Outlook is different from the Microsoft account that actually accepted the Family share. In that case, repeat the share process using the exact Microsoft account that is used for that mailbox.

References:

• How to share your Microsoft 365 Family subscription
• Troubleshoot sharing a Microsoft 365 subscription
• Share your Microsoft 365 Family or Premium subscription
• Microsoft storage quotas
• how to download microsoft office 365 from subscription with multiple installs - Microsoft Q&A