There is a statement in the closing credits: "No raccoon or tree creatures were harmed during the making of this movie. However the same cannot be said for the handlers of the raccoon and tree creatures."
Several credits in the closing credits read "I am Groot" before flickering and changing to the actual credit.
Zardu Hasselfrau's Guardians of the Galaxy: Inferno (2017) song is heard in the closing credits. At the end of the credits the disembodied head of Zardu Hasselfrau states: "In times of hardship, just remember: We are Groot!" just as in the video.
Cosmo the dog, Howard the Duck and the Grandmaster (a cosmic being from Thor: Ragnarok (2017)) are seen in the credits
SPOILER: There are 5 scenes in the closing credits:
The opening credits are of Groot dancing to Star-Lord's music, during a battle between the Guardians and a space monster (seen in the background).