If assigning individual permissions to the user fixes the error,
then the windows integrated groups that you've assigned does
not have the right permission assigned; make sure that each
individual user(s) is a member of that group.
In SQL Enterprise Manager give the Windows group permission(s)
to access the database(s).
From Microsoft:
When granting a Windows NT 4.0 or Windows 2000 local or
global group access to connect to an instance of SQL Server,
specify the domain or computer name the group is defined on,
followed by a backslash, and then the group name. For example,
to grant access to a global group called SQL_Users, in the
LONDON domain, specify LONDON\SQL_Users as the group
name.
To grant access to a Windows NT 4.0 or Windows 2000 built-in,
local group, specify BUILTIN instead of the domain or computer
name. To grant access to the built-in Windows NT 4.0 and
Windows 2000 local group Administrators, specify
BUILTIN\Administrators as the group name.
Here is a helpful link:
http://msdn.microsoft.com/library/default.asp?url=/library/en-
us/adminsql/ad_security_47u6.asp
