I recommend the Pentel Gel Roller Pen for Fabric. The ink flows very nicely, but you may need more than one pen. I suggest you send scrap fabric, so each person can "get the feel" of the pen before using and send extra fabric for mistakes. Search the net for good buys. I've been able to buy a box of 12 for $1.68 per pen.
http://www.pentel.com/store/gel-roller-for-fabric