AD group cannot connect to SQL from excel We have a data warehouse and users are trying to browse the database directly from excel. We put those users in an AD group and give them read only permission to a couple of databases.

The problem is under such circumstance users are not able to establish connection from excel to SQL Server. "login failed for user ***" will be shown as error message.

However, if we add every single user's login to SQL Server security, they are able to access to databases from excel then.

Does anyone have any idea why this happens?
Ah group permissions in SQL Server have never worked out for me right off the bat. I would check this question and make sure you are setting it up right on the SQL side https://stackoverflow.com/questions/5029014/how-to-add-active-directory-user-group-as-login-in-sql-server 

Also I agree with John, if you add a member to a group they need to log off and back on again before AD will update with the new group.