What i want to say

Perfume

it's a cute group their songs are very up beat and happy my favorite would be VOICE
 please check them out if you have time.

current members
Ayano Ōmoto
Yuka Kashino
Ayaka Nishiwaki

andymori

my next suggestion would be andymori i have just recently started listening to their music though it is a bit slow for me i still find  the music beautiful  my favorite of their songs is CITY LIGHTS if you want to go check them out at their official site.

the current members include
Sōhei Oyamada (vocals+guitar)
Hiroshi Fujiwara  (bass)
Kenji Okayama (drums)

KAT-TUN

my first j-pop suggestion would have to be KAT-TUN though recently one of their members Akanishi jin has left the band in order to go solo in the U.S  they still continue to be a band and create music
check them out if you have time they have a variety of songs though my favorite would have to be SIGNAL.
The name of the band KAT-TUN stands for first letter of the members names since the A (Akanishi) left the band, the (KA) in KAmenashi now stands for the KA in KAT-TUN
they also have a tv show called Cartoon KAT-TUN  though the show has already ended it was a great way for fans to get to know the members of KAT-TUN,

the current members include
Kazuya KAmenashi
Junnosuke Taguchi

Koki Tanaka
Tatsuya Ueda
Yuichi Nakamaru