Joining group is also the best way to level up.
Sometime you just nothing and you still level up.
Find a group with the most members and the KO sharing is 100%.
Then make sure you go to midtown (a location in the game) and wait there.
If any of the members KOed something you will received experience point.
Take note that when you KOed someting, you exp will also be shared to others.
If you decide not to share, just unjoin the group.
Thats all for today.