I am looking for an expression to describe the following type of people: a member of a group that, by his/her sole presence, makes other members more actively participate in the group activities.
More precisely, this person:
- does not need to be a designated leader or manager, like a class representative, or a club president, or a football team captain.
- does not directly push people to participate. His/Her sole presence and his/her behavior more or less consciously induce more engagement from other people.
- does not need to be loved to induce that effect. It is not necessarily a matter of competition.
- does not need to be better than other members of the group at what they are doing.
- does not need to be aware of his/her effect on the group.
Basically, for some reason that does not matter here, the group works less efficiently when this particuliar individual is absent.
For example, a student that asks many questions or propose ideas (good or not, it does not matter) in class and that entail other students to speak and participate.
French expression for this situation would be "Il est le moteur du groupe", but "he is the engine/motor of the group", although understandable in English, does not seem to return many results on search engines.