After the only male member of the group left for military service, Sunny Hill operated as a 4 person team consisting of Joobi, Seungah, Misung, and Kota. Searching for a member to replace their missing 5th person, several guest members were featured including Kim Jaewook and MBlaq's Chundoong. 

In a recent interview, the group explained "the guest members always worry about messing Sunny Hill up. But on the contrary they've always done so well and made our group that much more dynamic. So dont get too anxious about, it's not a burden!"

Out of all the guest members, however, IU stood out as she wore a black suit on stage (Jan 29th SBS Popular Songs) and performed along side Sunny Hill. Kota further praised IU for "monitoring if everything's going well and even secretly promoting Sunny Hill."

"She's such a wonderful sister. We can't help but love her."