Do you want someone local in Mexico? Do you want someone who will assist you in an active management capacity? Or do you want someone to provide you with boatloads of money so that you can hire more staff? One size does not fit all, so you want to get clear on what you need to accurately assess whether your potential "partners" can provide it.
Friends don't necessarily make for good business partners. You will want to carefully screen anyone you let into your business. Ask for credit reports and do background checks. And, yes, even with your friends.
In fact, especially with your friends. If you want to maintain your friendship, be even more vigilant about handling your business dealings and a professional manner.
Beyond that, be sure that you have a business entity that can accommodate different categories of owners. An S Corp will not give you the legal flexibility you need -- bringing in passive investors is best accommodated through a limited liability company or a C Corp.
And always have it done in writing. A qualified business attorney can help you navigate the variety of business partnership issues you will want to address.
Related: How to Find the Right Investor for Your Business
Related: 10 Steps to Organizing an LLC