Hi Elisabeth. A way to do it:
Make a new square composition (say 250x250). Double click the Ellipse Tool to create a circle - fill white, stroke pink. Keyframe the Elipse 1/Transform/Scale from 0 to say150 - so that the circle appears, grows and the composition ends up totally white.
Duplicate the square composition IN THE PROJECT PANEL and open this version of the square composition. Make the fill an alpha, so you have only the pink stroke.
In your main composition have the video you transition to above the video you transition from.
Add the white circle comp above the beginning of the transition. Go to the time when the circle just fills the square shape and then scale the circle comp so that the circle is the size you want them all to be.
Add the CC Repitile effect to the white circle comp and expand the up/down etc headings so that the screen is filled with the circles. (You may need to play with the scale of the comp to get even rows.)
Set the TrkMat of your incoming clip (the one below your white circle comp) to Luma.
Now when you play through the transition should basically happen.
Add the Stroke only composition directly above the circle comp and copy the CC Repitile effect to it.
You should have in layer order:
Stroke only composition
Incoming clip (with its track matte set to Luma)