Start it up, click Generate, and RanPassUI will happily generate a new password. Easy!
Simply add users in colon-separated "Username:User's name" format using the Edit Users dialog. The dialog will also let you import pre-existing text files and append them.
RanPassUI will then use these records to fill in template variables in your Prefix/Substitution Text.
Why not automate a useradd script which also populates a data file you can use later to notify them.
There are all sorts of possibilities. Examples (including useradd for Windows, Mac OS X and Linux/Unix) are available in the main menu under 'Edit → Substitution Text examples...' for your convenience.
RanPassUI is strong, powerful, yet provided at no charge. Hope you find it a useful addition to add to your toolkit :-).