If using Microsoft SQL Server 2005, 2008 or 2012, all users who are members of the sysadmin server role can create data in the same schema: the DBO schema. However, be aware that placing all the users in the sysadmin role means all those users are now administrators in the SQL Server instance and all the databases on the instance. This is an increased security risk and, therefore, is not a recommended practice.
Any user who is not sysadmin (and not DBO in the database) must create data in a schema that has the same name as the Windows login with which they used to connect to the SQL Server instance, for example, domain_name\user_name.
Using Windows groups is not a workaround. Placing all Windows-authenticated users into the same Windows group does not cause all data to be loaded or created in the same schema because Windows groups cannot have their own schemas. See the Microsoft site in the Related Information section below for an explanation of how schemas work for Windows groups.
Users connecting through a Windows-authenticated group do not have a default schema association. If such a user creates an object that is not qualified with a schema, a new schema is created, its name is set to the current user's name, and the table object is created in this new user-named namespace.
In geodatabases in SQL Server, each member of a Windows group has their own schema with a name that matches the user's login name. This match between the user name and schema name is required by ArcGIS.
For information on the geodatabase administrators and schemas, see the section entitled 'The geodatabase administrator and schema' in the Web Help topic The geodatabase administrator in SQL Server.
For information on how to use Windows groups with geodatabases in SQL Server 2005, 2008 or 2012, read the following section: Windows groups in geodatabases in SQL Server.