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Dear Haniff Hanifah,
Good day!
Thank you for posting to Microsoft Community. We are happy to support you.
Based on your description, I understand that you have a concern that when you just added 4 members into Sharepoint group and click 26 members to check but the pop-out appears showing the group only has 22 members.
May I know where you added these members, on Outlook or Microsoft Admin Center?
If on Outlook, when you added it, do you got a success prompt?
If on admin centerhttps://admin.microsoft.com/, you need administratorprivileges:
(If you are not an administrator, we recommend that you contact administrator to check)
If not, at once we suggest administrator that kindly remove and add again member into that Office 365 group from an Office 365 admin centerhttps://admin.microsoft.com/, try to identify the result outcome after some period of time (approx it may take 24 hours for taking effect from server).
Here is the detailed steps information article: Add or remove members from Office 365 groups using the admin center
Kindly feel free to contact us so that we can do further analysis and assist you. Your patience and cooperation are greatly appreciated.
Happy new year!
Best regards, Winter | Microsoft Community Moderator