a person who proposes a plan or idea first
The last character ペ is said to come from 屁 (へ, “fart”) and allude to the fact that a person who says, “Did someone fart?” is usually the culprit.
It’s usually used when you state that a person who proposes something first should take the lead and become in charge of it. This concept is called “言い出しっぺの法則” (“The law of iidashippe“) and there is even a Wikipedia article for it; however, this term itself is not commonly used in a conversation.
Since he is the person who brought up the plan first, he should organise the drinking gathering.