Very similar premise, about four best friends finding love for the first time in high school. Biggest difference would be how in Koi ni Mudaguchi every character has a partner, while HaruDan has a bit more drama between the boys.
Both dramas have a school setting with romance for each main character. The stories presented have comedy that all ages can watch, and no major conflicts exist. Each character has their own development, and their friendship is well depicted.
Both feature on a school club (of mostly boys) earnestly doing silly missions and have a similar madcap "suck the marrow out of life" vibe. Not finished either so that's all I can say for now.
