Edmodo is a private Twitter. You may not know what Twitter is, but you can find out by going to it. Twitter is good in several ways: You can speak in short sentences; you can chat; you can have instant access (through phone); you can connect to anyone in the world instantly; you can restrict connections to people you are most interested in. I use Twitter to learn what technology can do for teachers and classes.
The problem with Twitter is that your students may already use it, and, anyone can use it and even will come and find you, or maybe bother you, through your other connections or even through your students' connections. So, Edmodo is a way to have the benefits of Twitter but to have some control, so that only you and your class has a certain space.
How do you use it most effectively? I say, first try it with other teachers. Become comfortable with it. Learn how to sign in and sign them in.
For our edmodo, go to Edmodo and sign in as a student with access code xly289. Our group is called "Arequipa". That's all you have to know. You may have to have a logon name; if you speak, we will know you by that. Participate. Do the assignment; tell us what you think.