I found out that in order to assign a user from Aha to Trello you can use Tags to let Trello users know that a card is assigned to someone. However it is not really assigned to someone, it is just a tag( Capture5.png #2). Once in trello you have to manually assign the card to someone (Capture5.png #1). This is not such a big deal but it would be a very nice addition.
A workaround to assign someone to a task is to create a "requirement" inside a feature (assuming you have previously mapped a feature to a card).
Currently you can map a “requirement” from Aha! to a “checklist” in trello (Capture5.png #3). This is indeed a very nice feature between Aha! and Trello. However, I believe that the correct way to map this would be:
Currently there is no way that I can map a “To do” to a checklist. You can only map a "requirement" to a "checklist". Could you add this? (Capture6.png). This would be a good feature to assign a task directly from Aha! to someone in Trello (as I will explain next).
Here’s the good part: You can actually assign a Trello user directly from Aha! The only catch is that you have to know the Trello username in order to assign something to him/her from Aha! You can do that directly from Aha with an “@” and its username(Capture7.png).
So a good integration feature would be to map the usernames in Aha to the corresponding usernames in Trello, this way we can have an almost complete integration! ;-)
This way we could assign a “To do” from Aha to a “Checklist” in trello along with the responsible person.