Group with no owners

A Microsoft 365 administrator must assign you as an owner because an ownerless group cannot be claimed directly by a member in Outlook on the web.

If it is a Microsoft 365 group

Ask a Microsoft 365 administrator to:

1. Open Microsoft 365 admin center.
2. Go to Teams & Groups → Active teams & groups.
3. Select Filter → Ownerless groups.
4. Select the affected group.
5. Assign your account as an owner.

Alternatively, an administrator can use Exchange admin center:

1. Go to Recipients → Groups.
2. Select Filter → Ownerless groups.
3. Select the group.
4. Choose Add group owners, add your account, and save.

These are the documented recovery paths for ownerless Microsoft 365 groups. Groups require an owner to change settings or manage membership.^1^

An Exchange administrator or Groups administrator can also configure an ownerless-group policy under Microsoft 365 admin center → Settings → Org settings → Microsoft 365 Groups. The policy sends invitations to active members asking them to accept ownership. This policy applies only to Microsoft 365 groups and has subscription or Azure P1/P2 requirements.^1^

If it is a distribution list

An Exchange administrator can assign an owner through Exchange admin center:

1. Open Recipients → Groups → Distribution list.
2. Select the distribution list.
3. Open Members.
4. Add your account under the owners section and save.

Each distribution list must have at least one owner, and the Members section is where owners and members are managed.^2^

If the admin center refuses the change because nobody is currently listed as a manager, an Exchange Online administrator can assign ownership with PowerShell:

Connect-ExchangeOnline

Set-DistributionGroup -Identity "Group Name" `
  -ManagedBy "[Email]" `
  -BypassSecurityGroupManagerCheck

Get-DistributionGroup -Identity "Group Name" |
  Format-List DisplayName,ManagedBy

The administrator must be an Exchange Online admin, Exchange admin, or have the applicable Security Group Creation and Membership role to bypass the current manager check.^3^

If the group is synchronized from local Active Directory

If its properties indicate that it is synchronized from on-premises Active Directory, ownership must be changed in the local directory rather than Microsoft 365. The local Active Directory or Exchange administrator should update the group owner and then allow directory synchronization to send the change to Microsoft 365.^4^

If you do not hold one of the required administrator roles, provide the group name and your account address to your organization’s Microsoft 365 or Exchange administrator and request that they perform the matching procedure above.

References

1. Manage ownerless Microsoft 365 groups and teams
2. Create and manage distribution lists in Exchange Online
3. (You don't have sufficient permissions) error when you try to remove or make a change to a distribution group
4. Hello, I can´t add members to an existing mail group of my ownership - Microsoft Q&A