I did this for my parent's 50th wedding anniversary. I purchased the Pigma Micron pens and actually sent them with the fabric squares. Luckily my dad's family was concentrated in one area. So I sent squares and pens to an aunt and asked her to get them signed.
I also recommend putting blue painters tape around the edges to 'frame' the area to be signed - so no one writes in the seam allowance.
You need to specify that ONLY those pens can be used. Even with that instruction someone signed one square with a different pen. When it got wet (with spray starch the writing bled.
I do not recommend the Sharpie markers. I have seen where they are NOT recommended for fabrric. This is what it says on the Sharpie website:
Why does the Sharpie packaging say it is not for letter writing or cloth?We do not recommend the Sharpie for letter writing as it is solvent base ink and will soak through paper. We also do not recommend the Sharpie for use on cloth as it may fade or bleed in the wash. Also, over time a yellow halo may appear around the marking.