Usage: UMODE i
Usage: MODE <nick> +|-i

i - Invisible:
Makes a user invisible, you will not be able to see what rooms this user has joined unless you are in that room with the user. Most people set themselves +I to avoid being randomly spammed or followed into rooms.

If the server is running in Super Invisible mode then a more strict version of the +i user mode is used. When this is enabled (it is by default) the server will not allow users to message other users that are also +i that do not meet the following criteria:

1) They don't share the same channel.
2) They don't have a watch for the other user. (Both sides must have watches set).
3) The user is not a buddy.

Privilege: User

/mode Dan +i
/umode +i