In the upper right corner of the Ponder site, click "Settings".
In the settings for each of your classes, you will see a button called "Invite".
When you click the button, you will be presented with two options in a drop-down menu: "Readers" or "Co-Admins". Readers do not have access to the administrative tools for a class - this is typically the invitation you want to use for your students. Co-admins have the same controls over the class as you do - this invitation would typically be used for co-teachers of your class.
Once you make your selection, you will see a sample mail with a registration link unique to your class. Anyone who visits the link, will be prompted to register for Ponder, and when they do they will be automatically connected to your class as either a reader or a co-admin. If you don't have all of the email addresses of the people you'd like to invite, or for some other reason cannot email them directly, post a link on your class website, or create a bitly link to write on the board in your class. Keep in mind that Ponder is designed for users to be in multiple groups.
When students visit the above link, they will be sent to the student registration page, which will walk them through install once they have registered.
Students can use existing Google accounts to login and register with Ponder too.
We can also pre-create accounts for your students if that is easier for you.