TL;DR : Ultron asked the Twins to meet him at the church. We don't see that on-screen, though.
Although not shown on-screen, the implication in the dialogue is that Ultron summoned the Maximoff twins to meet him at the old church. When the twins arrive, the first line of dialogue from Wanda emphasizes this:
WANDA: Talk. And if you are wasting our time...
ULTRON: Did you know they put this church in the exact center of the
city? The Elders decreed it so that everyone could be equally close to
God. I like that, the geometry of belief. [pauses] You're wondering
why you can't look inside my head."
WANDA: Sometimes it's hard. But sooner or later every man shows
ULTRON: Oh, I'm sure they do. [stands to reveal himself] But you
needed something more than a man. That's why you let Stark take the
WANDA: I did not expect... [looks Ultron up and down] But I saw
Stark's fear. I knew it would control him, make him self-destruct.
ULTRON: [pacing around church] Everyone creates the thing they dread.
Men of peace create engines of war. Invaders create Avengers. People
create... smaller people? Er... children! [laughs] I lost the word
there. Children. Designed to supplant them. To help them end.
WANDA: Is that why you've come? To end the Avengers?
ULTRON: I've come to save the world, but also... yeah.
Given Ultron's control over both the Internet and wireless devices, the most likely form of summons was either an email, text message, or phone call.
Note: We also learn later in the film that Ultron chose the church for a very specific reason:
its location in the exact center of town makes it an ideal location for his device, which creates a spherical force field around the town.
Therefore, upon additional viewings (where we know what's coming), this meeting location - as well as Ultron's speech while there - takes on a more ominous tone.