How to add a new person who does not have a SMARTY Sim to my group

I would like to add another person to my group.   He is currently neither on Smarty/Three network so I presume I need to order a new SIM for him but I cannot seem to find where to do this.  I know which plan I want but there is nowhere to click to order a new SIM for a  new person.   Am I the only person who finds the group management complicated.