Within the canon it was not uncommon for the Emperor to bring several of his previously discovered sons along when meeting a new Primarch. I believe when he met Konrad Cruze he brought 3 or 4 in fact. 

Since no timelines have really been established, though you've made it clear Groseval was last, I see no reason why the Wild Blades cannot have joined the Crusade and probably been active for many years before his discovery.