I waited 48 hours for the sealer to dry thoroughly before I grouted the top set. When I bought the bricks, I also purchased grout. For the top set, I used "Miniature Brick & Stone Mortar #OR599W-White" I used a foam brush to apply it and get in between the bricks. Then I used another foam brush that was damp from water to brush the grout off the bricks, rinsing & blotting the brush as needed. This grout is sandy & gritty, but it didn't harm the sealed bricks. And the grout texture seems to add to the realism. Although the grout looks bright white in the photo, it is a creamy white in person.
In this photo, the bottom set of bricks has one coat of the same matte sealer I used for the first set of bricks. But these were from a different bag of bricks and it shows how the brick color varies. When doing a large project that will require more than one bag of bricks, I suggest you use bricks from both bags as you go along so it looks consistent.