"Als jullie gaan zwemmen, dan doe ik mee."
Translation:If you are going to swim, then I am joining.
Probably because the verb meedoen means to 'to take part in', 'to join' or 'to participate.' http://www.vandale.nl/opzoeken?pattern=meedoenlang=ne. "If you go swimming, I go with you" can be translated as "Als jij gaat zwemmen, ga ik met je mee."
How come that "then I join" is not accepted, while "then I'll join" is not? I agree that "I'll join" is more natural English, but is this really the direct translation? No trainings up until this point has touched on future tenses?