I want to set up a closed Facebook group for a group of professional colleagues to communicate and collaborate on a project. But most of these people are not already Friends on Facebook, nor will they likely want to be. I gather we can invite each other via email, but my central question is: once we're all in a closed group, does that mean we can see personal info and posts about each other that are on our respective individual FB pages? If the answer is yes, I need to find another mechanism as people will understandably not want to participate in the group.
What you can see on a person's Facebook profile all depends on that person's privacy settings. If you are not friends with them, then you won't see anything that has a privacy setting of "Friends", but you will see things that have a privacy setting of "Public" or possibly "Friends of Friends" (if you are friends with their friends). The fact that you are in the same closed group as them has nothing to do with what you can see on their profile.
I verified this by looking at the profiles of various people (who are not my friends) in the same closed group as me. The only things I could see on their profiles were things that they posted with a privacy setting of "Public".