Reasons why you could be having the problem:
You don't have your port or IP settings set right in the SPHERE.ini.
You don't have the ports for UO opened correctly
You are giving out the wrong IP or port to your friends.
You didn't make your friends an account or they are using the wrong client version.
Theres probabily a few others but those are the ones I can think of at the moment.
Make sure you have the settings at the bottom of your SPHERE.ini set to what you are using.
Your Sphere's Name Here
Open ports 2593 and 2595. If you changed the port in the SPHERE.ini then open the port you are using and 2595.
If you go through everything and it still doesn't work, you will need to give more information then what you have posted. I'm pretty much shooting in the dark here.