To add sessions on the Run Leader app, go to My Groups and then select the run you would like to add sessions for. In the header you will see a button that says 'Add session'. Select this and once you have added all of your required dates, select 'save'.
Alternatively, in the Schedule tab, select the + icon and select 'Add a session'. Select your group and run, and add the date of your session. When ready, hit 'save'.