What does it mean to Group your nick and how do I use it?

Answer 1

GROUP registers your current nickname to your account. This means that NickServ protects this nickname the same way as it protects your account name. Most services commands will accept the new nickname as an alias for your account name.

Note: Please note that grouped nicks expire separately from accounts. To prevent this, you must use them. Otherwise, all properties of the account are shared among all nicks registered to it.

 

/msg NickServ IDENTIFY OldNick SecretPassword

also you can try
/msg NickServ GROUP

Answer 2

/msg nickserv group MainNick MainNick'sPassword

group allows you up to 10 nicknames together under one password allowing you to switch between them once you're identified without needing to identify again.