Custom aggregate function

hi how do i write a custom aggregate function or select query.I have Table name MemberDetail where which has two column Memberid and WokingDay.In this table same member could have multiple working days. i want select query that will return single row for each member and his/her quama separated working days. without using cursor. how do i do that.

eg

Table MemberDetail

MemberID WorkingDay

M1234 12

M1234 10

M1555 02

M1555 15