Powershell Import/Export-csv group & join cells

I have a csv file which I need to group and join various cells together and have them then separated by a semi-colon in the csv layout is as such

CSV Layout

I have managed to join the cells 'price' & 'functionnumber' together and separate them with a ';' but for 'datedespatched' and 'username' which just need to display the 1 name or date the cells return with 'System.Object[]' displayed in the condensed cell, the 'referencenumber' cell appears to condense down fine. This is my current code and the output I receive from it

 Import-Csv 'C:\temp\test.csv' | Group-Object ReferenceNumber |ForEach-Object {
 
 [PsCustomObject]@{

        ReferenceNumber = $_.Name

        userName = $_.Group.userName

        DateDespatched = $_.Group.DateDespatched 

        functionnumber = $_.Group.functionnumber -join '; '

        Price = $_.Group.Price -join '; '
        
        }
 } | Export-Csv 'C:\temp\test-export.csv' -NoTypeInformation 
 

Code-output

Thank you in advance

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