I think the problem is in the number of parameters. The function accepts two and you are passing three.

https://docs.aws.amazon.com/redshift/latest/dg/r_CONCAT.html

== Redshift now has support for this as answered by rohitkulky, do not use this hack ==

Looks like there is no straight forward way to solve this. Here is what i did to solve it, this solution works only when u know how many times ur group by field is repeated e.g in above case its 2, as company_id is being repeated twice. In my case i know this count so this solution works for me, although not very elegant

If group by count is 2

select e1.company_id, e1.name || e2.name
from employee e1, employee e2
where e1.company_id = e2.company_id and e1.id < e2.id;

If group by count is 3

select e1.company_id, e1.name || e2.name || e3.name
from employee e1, employee e2, employee e3
where e1.company_id = e2.company_id and e1.company_id = e2.company_id and e1.id < e2.id and e2.id < e3.id;