Who qualifies as an inventor? Under US law, an inventor is someone who contributed to the conception of the invention claimed in the patent application. "Conception" is defined as "the complete performance of the mental part of the inventive act" and it is "the formation in the mind of the inventor of a definite and permanent idea of the complete and operative invention as it is thereafter to be applied in practice." See Townsend v. Smith, 35 F.2d 292, 295 (CCPA 1929). I will talk more about what is required to show conception next time.
Shifting our attention back to what makes someone an inventor, it is important to note that when conceiving the invention, the inventor can adopt general ideas and suggestions from others (i.e., conduct research) as long as the inventor maintains intellectual domination over making the invention. Morse v. Porter, 155 USPQ 280, 283 (Bd. Pat. Inter. 1965).
This can get a little hairy when working in teams or groups because everyone participates in brainstorming sessions and it can get a little bit difficult and confusing to keep a track of what each person said or did. (Tip: take notes) Not everyone in the team may qualify as an inventor though. Someone who merely provides a general knowledge or suggests a desirable result does not qualify that person as an inventor.
For example, let's say Alex is building a mobile app for finding dog walkers. Then Charlie comes along and suggests that the mobile app should alert dog walkers when they are hired or booked. Alex adopts that suggestion and figures out a way to enable the app to send alerts to the dog walkers when they are hired. Here, Alex would be the inventor, but not Charlie. (For chemical patent applications, an inventor must conceive a specific compound being claimed in the application.)
Another thing to note about inventorship in the US is that inventors are not required to reduce the invention to practice. But a lot of people don't know this and they are concerned about not having a prototype or a beta version of their invention. (You don't even need to have a prototype in order to file a patent application.) Also, someone who reduces the invention to practice is not necessarily the inventor. Let's go back to the mobile app example that I illustrated above and say that Charlie ends up building the mobile app, not Alex. Alex is still the sole inventor of the invention, even though Charlie built the app. But if the scenario was modified so that Charlie actually figured out a way to enable the app to send alerts to the dog walkers when they are hired, then Charlie would be one of the inventors.
This brings me to my next point about joint inventors. Joint inventors don't need to: 1) physically work together; 2) make the same type or amount of contribution; or 3) make a contribution subject matter of every claim of the patent. This means that Alex and Charlie can be co-inventors even though Alex makes 70% contribution and Charlie makes 30% contribution. Moreover, joint inventors are presumed to have undivided equal partial interest in the invention as a whole, even though each inventor makes different amount of contribution.
Errors in specifying inventorship may be amended but this can be a pretty complex process so it's best to make sure that inventors are correctly named when you first file the application. When in doubt, take a look at what is being claimed and determine which inventor can be credited for each of the claim elements.