When someone has great influence within a group, what do you call his situation?

clout

[MASS NOUN] informal Influence or power, especially in politics or business:

He's a person of stature / standing within the company.

influence noun

synonyms: . . . status . . . power, authority, sway, leverage, weight, standing, prestige, stature, rank, ranking, position, social position, station, connections, contacts;

[The dictionary at Google]

You might be thinking of gravitas

I'll just take a moment to recommend Iain M Banks' scifi novels and note his long-running joke on spaceships' names that use and abuse "gravitas".

You can say he shows "leadership".

• leadership - "the power or ability to lead other people" MW

"He showed strong leadership. That's why he was promoted."

• lead - (verb) -"to direct on a course or in a direction"
• leader - (noun) "one who has influence or power" - The Free Dictionary

Edit - As you have edited saying you don't mean someone who leads, but somenone who is experienced, influent, and whose opinion carries a lot of weight, I think you mean "a management guru".

• guru - "a person who has a lot of experience in or knowledge about a particular subject". MW

• An influential teacher or popular expert: ‘a management guru’ Oxford Dictionary Online