Can I have external people proof-read texts from teammates in a group project?
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.