If you are attempting to create a delimited list of values from the same column but different rows, take a look at "coalesce". Here is a simple example that concatenates ChildIDs for a specific ParentID:
create table RelationshipTable (ParentID NUMERIC, ChildID NUMERIC)
-- insert some data into RelationshipTable
declare @MemberList VARCHAR(7000)
SELECT @MemberList = COALESCE(@MemberList + ',', '') + RTRIM(ChildID) FROM RelationshipTable WHERE ParentID = 12345
Hope this helps
Wayne