dt <- MAC_trans_sales_members
setDT(dt)[, fwrite(.SD, paste0("output_", join.cohort,".csv")),
      by = join.cohort, .SDcols=names(dt) ]

Error: is.character(file) && length(file) == 1 && !is.na(file) is not TRUE

setDT(MAC_trans_sales_members)[, write.csv(.SD, paste0("output_", join.cohort,".csv")),
by = join.cohort, .SDcols=names(MAC_trans_sales_members) ]

Error in file(file, ifelse(append, "a", "w")) :
invalid 'description' argument
In addition: Warning message:
In if (file == "") file <- stdout() else if (is.character(file)) { :
the condition has length > 1 and only the first element will be used

daply(MAC_trans_sales_members, .(join.cohort), write.csv)