Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Uproar Forest

In The Dungeon
Once you reach the 10th floor you will find the Mankey gang. Afterwards everyone will return to your rescue team's headquarters. The Mankeys will help upgrade your teams HQ in exchange for a spiky chestnut. A total of 2 chestnuts will be required to advance in the storyline. If you don't have enough to bribe the Mankeys go for a few more missions in Uproar Forest until you have found some.

Go to the bulletin and take the Smeargle's mission.

The Last Floor
Smeargle is on floor 15. After finding him you will return home and Smeargle will now join you.