Users called 'Guest'

Hi,

I'm sure that this is just a setting....but I've had 3 new users join in the last 24 hours...and they're all called 'Guest'!!!

When I look at their profile Guest is in the first name field, but I'm assuming that I've set it up wrong somewhere as I've just registered...filled my first and last name in ok, but on the user page my name says Guest. username's ok....just the real name!!

Any ideas please