does it last a number of rounds = to your int modifier or does it last 1 round? It says both in the description.
It lasts a number of rounds equal to your Int modifier, however if someone leaves the radius it lasts for one round and then ends for them.
It also sounds as though an ally who wasn't inside the area of effect on the round it was created can enter the field on a subsequent round, as long as it's still on, and gain the bonus. I think it would be simpler just to say that your allies have the bonus as long as they're inside the field, and lose it if they leave the area; that makes it slightly weaker, but not by very much, and would simplify the description.