"Joining" doesn't have any tense, so it can't be right. "Joined" is past tense, so it is the right answer. However, the sentence has another problem in that "they" is the subject in the first clause, but "a welder" is the subject of the second clause. It's not "wrong," but it would be better to use "welders" as the subject of the second clause so it is more logically connected to the first clause.