What legal steps should we take before releasing an app?

Although trademark protection is always recommended, I advise you to focus on what is a priority for you..."the app itself". I could not give you specific information about Software protection in the U.S. but generally speaking it is necessary to draft an End User License Agreement (EULA) where you can set the ground rules for user when they are using your app. In this EULA it is important to include terms for Privacy, Intellectual Property, Warranties, Updates, Minimum System Requirements, Cloud Storage (if it is the case), Technical Support, Disclaimers, and more.

Consult with an attorney(s) about (1) forming an LLC or Corp. and protecting yourselves from liabilities (e.g., if your group of developers infringes my patent, I may sue each of you individually), (2) protecting yourselves from additional tax liabilities, (3) protecting your intellectual property (app function, design, trademark, confidential information), (4) protecting yourself from each other/employees/contractors (e.g., what happens if one member leaves or steals the source code?), (5) allowing year company to grow, (6) EULA will likely be boilerplate similar to other apps, but certainly deserves some thought before release.

Basically, if this development team is anything more than just a group of people hanging out and coding for fun (no offense if it is!), you need an attorney to help walk you through some things. The biggest thing is to protect yourselves as individuals because if you're successful at all, you will become a target for lawsuit. There's a lot more to think about when discussing with an attorney about your specific details. Best of luck!