"In" vs. "of" after the superlative form of adjectives

Both in and of are possible after superlatives. ‘An A-Z of English Grammar’ by Geoffrey Leech and others explains that ‘usually “of” is followed by a plural noun, while “in” is followed by a singular noun.’ Of is perhaps more usual with team because, although it is grammatically singular, it represents a group of people.

I would definitely choose of. It is not a grammatical error to say 'he is the youngest member in the team', but the statement requires the question 'member of what'? It is a semantic matter. If you say 'he is the youngest member of the team' it is unambiguous.

In this case, I think of and in are both acceptable, but I would favour of.

In more general sense, a person or thing of a collective noun implies that the noun is constructed out of a collection of such members. Something in a noun is more general, the noun might exist separate from the people or things who are included in it.

When you say

He is the most committed person of the club.

you are emphasizing that the club is just a collection of people, while

He is the most committed person in the club.

emphasizes that the club exists separate from its members.

In most cases, either in or of is acceptable, but there are stylistic preferences. Member is nearly always followed by of because the word implies a constituent part of a group. Similarly, group is nearly always preceded by of because a group has no identity separate from its components. And of course, if the group is described using a plural instead of a collective noun, then always use of:

She was the youngest of the players.

Superlative adjectives wouldn't change the basic distinction between of and in, but the distinction creates the potential for different interpretations of an adjective.

She was first in the team on the racetrack.

implies that, in the ranking of all team members based on race results, she came in first.

She was first of the team on the racetrack.

implies she was the first person of the group to arrive at the track (although I would still recommend re-writing the sentence for greater clarity). I can't think of any similar confusion about the adjective youngest.

As mentioned by others in comments, "on the team" is another alternative, and probably more common than "in the team". However, that wording is really specific to team, perhaps as an extension of the idea of a team as a list of names of people who made the cut. You can describe a person as being "on a list", but not "on a group" or "on a class".

In my opinion, this is how you have to understand it:

He is the youngest (member of the team).

"Member of the team" stands on its own and can be considered as a single unit.

In addition, "member" cannot really stand alone. It has to be followed by 'of the team' to make sense and this unit must be considered indivisible.

Following the same reasoning, boy can stand on its own and this is why we can say: The youngest boy IN the team.

According to OPG Int. By John Eastwood we use in after superlative form with places and teams e.g. Pakistan is the most beautiful country in the world.