Marvin is the Groundskeeper of the mansion. He is not formerly introduced to all the guests in the beginning, meaning some guests did not get to meet Marvin before their fate.
An Invitation (Episode 1)
While the guests are eating dinner, Marvin puts an explosive under the wheel of the Get-A-Away Car that brought them to the mansion. It it not proven to be Marvin, as we never see anything but the hands of the person, but it is more than likely Marvin because of the similarity in the clothing.
Buried Alive (Episode 3)
Marvin is seen in the 'Months Before the Guests Arrived' sequence of Episode 3, digging the hole to bury Caroline Eastwick alive to conceal an artifact.