I am looking for a way to concatenate the strings of a field within a group by query. So for example, I have a table:
ID COMPANY_ID EMPLOYEE
1 1
This latest announcement list snippet might be of interest if you'll be upgrading to 8.4:
Until 8.4 comes out with a super-effient native one, you can add the array_accum() function in the PostgreSQL documentation for rolling up any column into an array, which can then be used by application code, or combined with array_to_string() to format it as a list:
http://www.postgresql.org/docs/current/static/xaggr.html
I'd link to the 8.4 development docs but they don't seem to list this feature yet.