I invested in a start up company as an angel investor. I would like to know who the other angels that invested in the company are. Can I do that?
I would question the motivations of the Founder/CEO if they are keeping this from you.
Pre-investment it's slightly different as it's understandable sometimes when negotiating if founders dont want to reveal other potential angels; you can then include that fact in your decision whether to invest. But post investment it would be very unusual and actually unhelpful to the founder/startup itself for investors not to know each other.
Ask for a copy of the CAP table.