What version of mIRC and Invision are you using?
As far as manually entering the information, you can easily see the possible addresses for a person in this way and then enter any of those into the User List...
//echo -a $address(nick,1)
Just use that and replace 1 with any number 1-20. Those are all the valid ways to enter that person's address. A ? mark is used as a wildcard for a single character and a * is used as a wildcard for a group of characters. If you find the type you like, you can then use that same number to get the addresses for everyone you want to add and copy/paste those in so you don't accidentally type them wrong.
Btw, 3 is a good number if their IP is dynamic, tho depending on how secure you want to be, you might choose something else.