This guest art bromide is by 小坂俊史/Shunji Kosaka with their series サイダースファンクラブ/'Ciders Fan Club' appearing. I've got 4 of the 5 characters that appear in it.
Medium black hair- Mera Shiwasu (I think)
Short black hair- Tomozawa Mutsuki
Long blonde hair- Nakano Yayoi
Long black hair- Oyama-san
Then it's just the long blonde hair girl's Junior (cat ears) that I don't know the name of
The Japanese Wiki has most of the characters name's along with the main 3's full names if anyone can help properly translate them