これを試してください;）
select
star.type, star.min_amount, t.cnt, t.userids
from star
inner join (
select t1.type, count(t2.userid) as cnt, group_concat(t2.userid order by t2.userid) as userids
from star t1
inner join user_buys t2 on t1.min_amount <= t2.amount
group by t1.type
) t on t.type = star.type
order by star.type