I can't modify the members of a group in Exchange Admin Center

The error “Failed to update members – Response Content is Null” when modifying group membership in the Exchange Admin Center (EAC) is typically a front‑end failure and not very descriptive. Based on the provided context, focus on these likely causes and workarounds:

1. Verify that the account has permission to change the group
   • If the account is not a manager/owner of the group (not listed in the group’s ManagedBy attribute), changes to membership can fail.
   • In Exchange Online or on‑premises Exchange, ensure the account is:
     • Listed as a group owner/manager for that distribution group, or
     • Assigned appropriate admin roles (for example, Organization Management in Exchange Online, as noted for similar membership operations).
   • If not, add the admin account as a manager/owner and retry the membership change.
2. Check if the group is very large (Exchange on‑premises, especially 2010)
   • In Exchange Server 2010, there is a known issue when editing large distribution groups (more than about 1,800 members) via ECP/OWA:
     • ECP incorrectly calls only Update-DistributionGroupMember instead of Add-DistributionGroupMember or Remove-DistributionGroupMember for large groups.
     • This logic defect causes membership updates to fail.
   • Resolution in that scenario:
     • Install Update Rollup 2 for Exchange Server 2010 SP3 (KB 2866475) on the Exchange 2010 servers.
     • After the rollup is installed, retry the membership change in ECP/OWA.
   • As an immediate workaround (if on‑premises Exchange is in use), use the Exchange Management Shell to modify membership instead of ECP:
     • Add-DistributionGroupMember -Identity <Group> -Member <User>
     • Remove-DistributionGroupMember -Identity <Group> -Member <User> -Confirm:$false
3. Use the EAC “Edit group” path correctly (Exchange Online)
   • In Exchange Online EAC, ensure the group is edited from Recipients → Groups, then select the group and use the Members section to add/remove Owners and Members.
   • For mail‑enabled security groups, use the specific Mail-enabled security tab and then the Members section on the group’s properties page to manage membership.
4. Try a different admin session and browser
   • If permissions and group size are not the issue, the null response may be a transient UI/API problem:
     • Sign out of all Microsoft accounts in the browser.
     • Clear site data/cache for the EAC URL.
     • Open a new private/incognito window and sign in again.
     • Retry the membership change.

If the environment is Exchange 2010 and the group has more than 1,800 members, installing the SP3 Update Rollup 2 and/or using PowerShell to manage membership is the most direct fix from the provided context. If the environment is Exchange Online, confirm group ownership/permissions first, then use the EAC “Edit group” → Members path or PowerShell as a workaround.

References:

• Cannot change a distribution group that contains more than 1,800 members by using ECP in OWA in an Exchange Server 2010 environment
• (You don't have sufficient permissions) error when you try to remove or make a change to a distribution group
• Use a screen reader to add or remove distribution group members in the Exchange admin center
• Create and manage groups in the Exchange admin center in Exchange Online
• Manage mail-enabled security groups in Exchange Online