Агрегаты и COUNT (Отличительный GameID):
declare @raw as table (GameID int, UsrRegID int, Score_user int)
insert into @raw values (281, 1, 1)
,(281, 1, 2)
,(281, 1, 3)
,(282, 1, 0)
,(282, 1, 0)
,(282, 1, 1)
,(283, 1, 2)
,(283, 1, 3)
select count(distinct GameID) as Distinct_Count_GameID, UsrRegID, sum(max_score_user)
from
(
select GameID
, UsrRegID
, max(score_user) as max_score_user
from @raw
group by GameID, UsrRegID
) a
group by a.UsrRegID