Whichever makes the most sense. I think this is one of those 'however you want to do it' type of things.
I'd bring them in on the edge of the tile it makes the most sense for depending on the Story.
Here is why i bring it up.
I'm playing on an epic tile 24x12. I have 4 serendipity tokens. I'm about half way through the tile working towards the other end. If I summon Marcus the Ready, and he enters where the heroes entered the tile, it will take him forever to make it to where the action is. If i summon him next to a hero that does not make sense thematically, It's like I just pulled him out of my pocket and place him next to a mini boss. Summoning him anywhere on the tile is just plain silly. The best solution would be to summon him to a any tile edge adjacent to the closest hero. But i'd rather this just be decided for me in the rules then figure out what makes the most sense.
I don't mind the "However I want to play it" answer, but i'd rather decide to house rule an official rule and play it how i want rather then just make rules that are incomplete.
Many questions i'm coming up with is anyones best guess, because there is no one (or few) in the player community that knows the answer or knows where to find it.
Hopefully the developers are keeping an eye on these threads, and will shed some light on these topics for those of us who are in the dark.