Why you need view and function?

What's wrong with a single query

select Id,ProductName,IsActive,

CONVERT(XML,

(SELECT TBL_Group.title, dbo.TBL_Group.id

FROM TBL_Group INNER JOIN

TBL_GroupProd ON TBL_Group.Id = TBL_GroupProd.GroupId

WHERE TBL_GroupProd.ProdId = TBL_Product.Id FOR XML AUTO, ROOT('main'))) AS [Group]

from dbo.TBL_Product

where ProductName = 'P4'

and exists (select null

from TBL_GroupProd

where TBL_GroupProd.ProdId = TBL_Product.Id

and TBL_GroupProd.GroupId = 5)