Can I have external people proof-read texts from teammates in a group project? I'm attending a seminar, where we've been tasked to read some papers, extract information for them, and write an essay about our findings. The lecturer has teamed me up with two other students for this assignment.

One of my teammates posted a draft of the introduction section for the essay. Upon reading it, it appeared to me more like a Vice article than a scientific paper, making it look like the student got the lecturer's statement wrong that "you are allowed to express opinions in an essay".

However, the thought occurred to me that my reaction may be caused by the opinion that the text voices. In other words, it may have been that the text appeared biased to me just because its opinion was not mine. I therefore gave the text to two friends of mine, and they arrived at the same conclusion as I did: The text was overly opinionated and did not provide enough proof for its claims.

Upon reporting my critic to my teammates, I have in turn been criticised for giving the text to external people without the knowledge and consent of its author. I do not concur with this statement, since I removed authorship information from the text before giving it to my reviewers, and since my reviewers cannot know who else is attending the seminar, so they will very likely not be able to deduce the identity of the author.

I call upon your moral judgment: Have I acted wrongly?
I assume that your teammate posted the draft to a place where only your team could access it. In that case, yes, you have acted wrongly.

Generally, you'd be expected to keep all research or project results from others at the level of confidentiality at which you obtained them. That means you have to get consent from the authors before sharing with anybody who didn't have access to this content before. That applies to project reports as well as later in your career to preliminary research results, manuscript drafts, or funding proposals.