Maple, Water Lily Bride, Viola, Nerine, and Cattleya already make a full party, so the game won't let you add another one.

Anyway, I agree with Wutan. It's not generally worth it. Attack up based on speed is a rather niche ability that doesn't really make an impact unless party speed is high enough to considerably buff everyone's attack. Among your girls, only Bride Water Lily and Nerine break 600 speed. Unless you gut out the lineup and use faster characters, you are likely to not get the most out of this from simply adding one of HTG, JSG, or Blushing Bride. I even find it out of place for the last girl because she lacks party speed up unlike HTG/JSG and is better off in a Solar Drive lineup. Characters like HTG and JSG are better off being the core to a specific lineup rather than being fillers IMO. Another thing is there are less than 5 of them with that exact ability (only 3 rainbows at that) in the game, so fielding an exclusive party with them isn't even possible – having this limitation for an ability is more or less a red flag for me in team building.