Here are 3 ways you could find out who the investors are.
At any AGM (annual general meeting, which you would have a right to attend) this would obviously be disclosed. Also in the shareholders agreement, all parties would likely sign and you could find out then, and without making a stink.
It depends on the jurisdiction but you could be able to see the public filings. This can be bad because generally it's not advantageous to have your ownership as public knowledge.
Why not just ask the entrepreneurs for an intro? You gave them money, surely they could introduce you to other investors...
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.
This should be a necessary part of your due diligence prior to completing your investment. Who else has invested, how much has been invested over what period of time? You should see a cap table and know what investment instruments have been used for prior investments (equity, either common or preferred; convertible debt/terms; debt/terms) before you invest, but if you already have, you should be able to get this information. My best advice? Ask!