Group by in DAX like in SQL
SUMMARIZECOLUMNS is the main function to use here.  

So this:

select SUM(Kiekis )
from Info
left outer join Prekes on Info.PrekeID = Prekes.PrekeID
left outer join PrekiuGrupes on Prekes.PrekiuGrupeID = PrekiuGrupes.PrekiuGrupeID 
group by PrekiuGrupesID


would be simply

   EVALUATE (
         SUMMARIZECOLUMNS(PrekiuGrupes[PrekiuGrupesID], "TotalKiekis", Sum(Info[Kiekis]) )
   )


DAX traverses the active relationships in the model, so there's no need to use explicit JOINs.  And instead of having a structured query language DAX uses function argument types to detect the different "clauses".  The first string argument to SUMMARIZECOLUMNS indicates the transition from "group by" columns, to pairs of (String,Expression) to specify the result columns.