I think you mean proof-reading. This is totally different thing from peer-reviewing. The peer-reviewing process refers to academics and experts in your fields giving you scientific feedback. This is usually offered through conferences, symposiums, and journals. Anything else is suspicious and you should not go for. While proof-reading is editorial feedback (grammar, spelling, etc.). It is up to your institutions rule whether to allow third-party proof-reading or not. You should look into your institution's rules. There are some who define what a proof-reader can/can not do, while some institutions prohibit this specially for students.
After some comments, it is clear to me now that you mean peer-reviews. As I said, this is suspicious. Respectful academics who can give useful review provide this service through known channels: conferences, journals, symposiums, etc.
Also, these channels ask these reviewers to confirm they will not disclose your work and they treat it confidentially. Other channels can disclose your work especially if you do not know the committee and if they do not have reputation to care about.
My advise: about my self, I will stay away from such shady channels. I only submit to legitimate known channels with clearly defined committee and rules. If something went wrong, there is a chair I can talk to. Otherwise you may find yourself in troubles.