To avoid the ambiguity you could express it in one of two different ways.
He was showing me additional pictures of those members of his orchestra who had appeared in the invitation brochure.
This sentence indicates that the pictures exist separately from the brochure itself.
He was showing me all of the pictures of the members of his orchestra that appeared inside the invitation brochure.
In this version, it's made clear that it's the pictures exist only inside the brochure.
Note that I changed on to in, assuming that if you opened up the brochure it would contain more pictures of the people. Idiomatically, in the brochure would also be understood to mean on the front and back covers. If the pictures only did appear on the front cover (perhaps as some kind of montage), then on could be retained—and my second sentence rewritten to accommodate that fact.