Hi @MrNguyen ,
Based on the issue that you mentioned, I have made a test, and the issue is confirmed on my side. Even though, the Office365Groups.AddMemberToGroup(...) formula add a user to a specific group successfully, it would still show up the "The data returned in the response is invalid" error.
Based on this issue, I think it is related to Office 365 service API. I would post this issue to my product team, if this issue is solved, I would reply here.
As an alternative solution, I think the AzureAD connector could achieve your needs. I have made a test on my side, please take a try with the following workaround:


Add a Dropdown (Dropdown1) in your app, set the Items property to following:
Office365Groups.ListOwnedGroups().value
Add two buttons in your app -- "Add User To Group" & "Remove User From Group". Set the OnSelect property of the "Add User To Group" button to following:
AzureAD.AddUserToGroup(Dropdown1.Selected.id, AzureAD.GetUser("Test2@Wicren.onmicrosoft.com").id)
on your side, you may type:
AzureAD.AddUserToGroup(Dropdown1.Selected.id, AzureAD.GetUser(_UserPrincipalName).id)
Set the OnSelect property of the "Remove User From Group" button to following:
AzureAD.RemoveMemberFromGroup(Dropdown1.Selected.id, AzureAD.GetUser("Test2@Wicren.onmicrosoft.com").id)
On your side, you may type:
AzureAD.RemoveMemberFromGroup(Dropdown1.Selected.id, AzureAD.GetUser(_UserPrincipalName).id)
Please consider take a try with above alternative solution, then check if the issue is solved.
Best regards,