1/4 is an equivalent fraction to 2/8. Thus, since you have 7/8, you can give at most 3 friends that 2/8 slice of pizza.
Why?:
3 friends recieving 2/8 slices would be a total of 6/8. If you were to give another friend a 2/8 slice, you would be giving away 8/8 of the pizza... but you only have 7/8!!!!! Therefore, at most you can give away 6/8 to a total of 3 friends, and then you wouldn't have any 2/8 (aka 1/4) slices leftover!!
Hope that helps, cheers