What do you call a person who does/thinks the opposite of the crowd for just that reason?

Such a person does not hold in high regard the decisions taken by a group (not a team) of people. She believes that groups tend to be anarchic and don't serve their purpose.

An example would be - for outings with friends, this person will not like planning it with the group since they will mess it up.

Therefore, this person has convinced herself that when more people are involved with something, it will be worse. She distrusts crowds instinctively basically.

Solution 1:

I'd call that person a

contrarian: a person who takes a contrary position or attitude

Solution 2:

I don't say contrarian isn't a valid answer, but it's not that common. People are more likely to say...

"He's just being contrary" rather than "He's a contrarian"

But far more common in my experience (particularly in speech) are the idiomatic...

"He's a dog in the manger"

...which calls attention to the fact that his refusal to go along with the majority detracts from other people's enjoyment, or is otherwise detrimental, and...

"He's playing devil's advocate"

...which accuses someone of arguing for a position they don't really endorse, simply for the sake of making an arguement.

I don't recognise the particular personality OP describes (would she be happy to accept a decision made by one other person? Would it make any difference if that decision affected just the two of them, or everyone else in a large group?). She might be any of...

[nonconformist, renegade, maverick, rebel, radical, individualist, heretic, dissenter, dissident, iconoclast, loner], etc., etc.

...depending on exactly what other characteristics she has, and on OP's value judgement of her.

Solution 3:

You could call that person an individualist.

individualist: a person who exhibits independence and individuality in behaviour, opinions, or actions

If you wanted, you could also use the term rugged individualist, a term with a most interesting Ngram. Here's an example of its use:

The value of competition and individual excellence is highly prized... The value of collaboration is less well understood and rewarded. As Norris stated: "In our society, which glorifies the rugged individualist and emphasizes confrontation, cooperation is the recourse of last resort... You cooperate when nothing else works." (Zonana, 1988)

You might also consider loner, or even lone wolf.

loner: someone who prefers to be alone and to do things without other people
lone wolf: someone who likes to be or work alone; a person who avoids the company or assistance of others – also called a lone hand