You're going to have to ask a straight question if you want a straight answer.
First of all, I've been playing fantasy sports since the early 90s, and I've never heard of "Free salary cap fantasy football." I can only take a guess at what you might mean.
In salary cap fantasy games, you form your team based on the price of the players (which is assigned by the website), and you cannot go over a salary cap. This means that your team may share the exact same players as someone else's team. And usually the prices change from week to week, so you usually have to use different players each week to fit the most fantasy points into your salary cap.
These types of games are the heart and soul of daily fantasy sports (DFS) and you will find them on sites like Fanduel and DraftKings. You can make a "league" and have your friends join, but it's not a league in the sense that you have standings, playoffs and a championship. You usually play head to head or within your group (league) to win a prize for that week.
I've also seen salary cap contests that have a large field of contestants competing for the most overall points on the season. You can make a private group (or league if you will) and have your own contest within that group to see who does the best).
But again, this isn't the same as forming league with 10 or 12 teams and everyone drafts players from the nfl onto their team and only their team if that is what you are asking about.
Hope I was able to answer your question.