Problem With Domain Group And Diagrams

I have a problem and I dont have an idea to solve it. I work with SQL Server and my students on Faculty of Information Technology.

I have create SQL Server login for domain group of users (about 60 od them). That mean I have SQL Server login like this DomainUsers
To whole group of users I had grant server role (dbcreator). I dont wish to import 60 login one by one and gice that permision.
Ok, student can create database without any problems
But, when they try to create database diagram (expanding database diagram node). They get a message that database need to have a valid owner (Hmm..ok its not a big deal to do it)
Quick check to some of database (Right click-->properties-->General-->Owner (ther is a DomainUserName (not name of group)..and that is ok
(Right click-->properties-->Files-->TextBox Owner. I try to enter same DomainUserName from step 5. But I get this message

TITLE: Microsoft SQL Server Management Studio
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Set owner failed for Database '1110_EvidencijaKnjiga'. (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=9.00.3042.00&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Set+owner+Database&LinkId=20476

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ADDITIONAL INFORMATION:

An exception occurred in SMO. (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=9.00.3042.00&LinkId=20476

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The login 'DomeinUserName' does not exist on this server.

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BUTTONS:

OK
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This is wired. Ok I know that that user is not login but group is. Step 5 show that user, but step 6 have empty owner options and dose not allowe me to enter same user.

How to fix this?

Thank you