Unless you have a tailwheel endorsement in your log book as a result of dual training with an instructor you do not have tailwheel differences. Unless your group member is a CRI ir FI then you have not been trained and can't log the time. Nir can you fly that aircraft solo or as PIC until such time as you have received the appropriate training.
If you had a tailwheel endorsement already in your logbook and were being checked by another group member for insurance then only one of you can log any time.