This command lists users who are on the network, regardless of whether or not they are registered. You can search for nicknames on the network as a whole based on wild cards or you can search all users on the specified channel. Note that although the official documentation states that this command can use perl compatible regular expressions, this is confirmed not to work in the current version of services. This command accepts 1 or 2 arguments, the pattern to search for, and an optional argument to search for users with user mode +i, the word invisible.
To search for all invisible users whose nickname starts with the word bot, you can run this command:
/msg OperServ bot* invisible