Hi @MMarius
The ones that you have there as variable are all of type string. If you look closely in the flow variable tab of the joiner you’ll see that it needs to be of type string list: s[] . In any case the actual flow variable type needs to align with what the node is expecting 
There are several ways to create this string list, depending on how you are defining your desired columns. Below is a small test setup:
See WF:
Multiple column names in variable piped in Joiner Included_names.knwf (32.4 KB)
Hope this provides some inspiration!