I started the process of entering the SFO but backed off due to the fact that right before I started formation my wife got pregnant. I figured it would be best to wait.
The Secular Franciscan Order is like the Third Orders of many other groups, but it isn’t THE Third Order. When I first started looking into it, this confused me. Third Order Franciscans are usually priests, deacons and nuns, similar to First Order Franciscans but they tend to live in the “real” world a little more then First or Second Order Franciscans. Many Third Order Francicscans are priests of diocesan parishes, work in schools, etc.
The Secular Franciscan Order is for lay people that want to follow the rule of Saint Francis, which can be found on the link that was already provided. At least in the group that I was involved with there is a Third Order priest associated with the group to provide spiritual advice.
The process for joining is basicially you first must attend 3 of their normal meetings (usually they meet about once a month.) So you can find out about the group. Then if you are still interested you enter formation. During this period you study, and determine if you are really called to enter. After the formation period you take the vows.
From what I found there are a very devote group, of course this like any other group could vary depending on the members of the organization in your area. There are many members of SFO online that can better explain what it is like.