Usage: BUDDY [ADD|DEL|LIST|WIPE] [<nick>]

This command will have NickServ manage your BUDDY list. A Buddy list is the same as a watch list and similar to a notify list. A Buddy list informs you when each of your buddies joins or leaves the chat network.

ADD - Add a Buddy to a user's Buddy list.
DEL - Delete a Buddy from a user's Buddy list.
LIST - List the user's Buddy list.
WIPE - Remove all entries from the buddy list.

/msg nickserv buddy add Mark
/msg nickserv buddy del Mark
/msg nickserv buddy list

You can add the nicks of people whom you frequently chat with as buddies. But you might not want to make your buddy list too long, or you may be seeing too many messages about people logging on and off. Add people to your buddy list if you will frequently want to chat with them when they are online and might not otherwise realize they were around.

Other related commands:
/msg nickserv help auth
/msg nickserv help set authorize