Sessions aren't added one by one. They come from your training days under Setup > Dates.
A set of dates
Give it a name, choose the squad, set the From and To dates, and optionally the grades it's for. Then add a slot for each training day: the day of the week, start and end times, the venue and the nets or areas you use. Every matching day in the range becomes a session. Add a break for the weeks you skip.
The cutoff
Each session's RSVP closes at a cutoff. NetCaptain sets it for you: four hours before the start, or 8pm the night before for morning sessions. If you want something different, set your own cutoff on the slot.
Players are asked from six days before a session, reminded the day before the cutoff, and again four hours before it if they haven't answered. At the cutoff you get an email summary.
Changes
A one-off session goes on the Sessions screen with Add once-off. If you edit a set so that a day disappears, that session is cancelled and its players are told.