micro pens are best, they are permenent, do not bleed or fade, are acid free and made for this.
we did one at my daughter's wedding reception i had made 300 siggy blocks...they were sitting on a table just inside the door of the reception hall with a basket of pens and a sign. everyone who attended their reception signed and dated a block (some people added little 'words of wisdom' for the new couple. then all of the blocks were put in a box for me to turn into a quilt! we used 3 1/2" siggy squares for hers.
and no...i do not have pictures...i was way worse back then than i am now...and i'm still bad about it but i do try harder now to remember to snap a pic.
you might want to make the blocks ahead of time...otherwise if you just send squares of fabric some people may write on them in ways you can not make them fit into your blocks without cutting off part of the name....or draw the area you want them to stay within---easier to just make the blocks i think