Are they suspicious? How could they not be? But the messenger does not bid them follow on the spot, only assemble in a meeting-place they know but have not visited, a small intimate pavilion whose price for one night's revelry is famously a thousand platinum coins. Even in the City of Brass where Efreet and Malik do vie to show their pride and wealth, it is a significant expense. With twenty such nights you could buy a Wish-diamond, even at its fair price in Brass from one Efreeti to another.
If it's a trick or a trap, it's an expensive-enough one that it would be dishonorable not to spring it, to leave their enemy forlorn at the altar after such grand preparation that honors them by this expense.