Try the correct order of statements:
SELECT oA.Id, oA/Class.ObjectA_User_Stakeholder_AT/Class.User/FullName AS ATMember, COUNT(oB.Id) AS NumberOfObjectB, oA.Column1, oA.ColummnB
FROM Class.ObjectA oA INNER JOIN oA/Class.ObjectB_ObjectA/Class.ObjectB AS oB
GROUP BY oA.Id