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Hi Prathamesh Patkar,
Thank you for your post in our forum.
Per your description, we know you encounter the error when you add an internal user into a SharePoint site.
Given this situation, we sincerely request that you could check the site permission. You can see the below methods to add the user.
- Open a site > Settings: Gearicon in the top right corner > Site permissions > Advancedpermissions settings> Grant Permissions> Invitepeople> Enter the email of the invited user > Share. And then the invited user will receive an email. You can see more information from Customize SharePoint site permissions.
- Sign in Microsoft 365 admin center (you can contact your IT admin use admin account sign in) > Groups > Active groups > select and click the group name of the site > View all and manage members.
- Open Outlook > select and click the group name > Addmembers > Enter a name or email address > Add. Usually, this only works on the Team site.
Hope this can help you. 😊 If you have any confusion, you can post back, we will continue to help you till you find a solution.
However, if all ways don’t work, could you please create a new site to add an internal member?
Meanwhile, could you please tell us which kind of the site? Team site or communication site?
Can you other users into the site? Or in other words, you can’t add all internal users into the site?
Best regards,
Cindy
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