I believe what they might mean is if you don't personally use a project, you might not be as committed/passionate about it as someone who uses the software in their daily life and cares about it as a user.

So it's not about finding one with lots of users and dropping in there if it is software you don't care about.

If you are just getting into using open source software though and have yet to find any that are important to you, then you can usually find lists of open source alternatives (from a user perspective) with a search fairly easily to get to better know what is out there and what you might be most interested in. (And lists like that for people to find software like that are generally helpful, I found lots of tools I wouldn't have otherwise known about through peoples collections of favourites/alternatives/etc.!)