I'd go for fusible web (sometimes also called fusible interfacing). Choose the type with paper backing.
Think of it as the fabric equivalent of double-sided tape:
You iron it to the back of the fabric you want to put on your t-shirts, cut out the shape, peel of the backing, position it where you want and iron again.
Now, a dedicated crafter would probably then run the whole thing through the sewing machine and add decorative stitching, but for a tennis match (assuming that's why you want FEDERER) or three, your iron-on letters will be more than sturdy enough. The webbing will prevent fraying and you can wash and probably even tumble-dry the t-shirts.
The main advantage is, that you don't have to worry about which paint or marker will be sufficiently opaque. A few yards of fusible web won't break the bank and you or another of your group probably have some old sheets or similar around, that you can cut up for the letters.