AD group cannot connect to SQL from excel
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.