The procedure is very simple: visit the Account Management section of the PlayNC website to manage your current account (you'll need your PlayNC master account name and password to log in).

You can find full instructions about how to send invitations with the following Dungeon Runners Support Center article: How can I invite my friends to play Dungeon Runners with me?

Note that you can send up to four invitations at any one time. When one invitation gets accepted (that is, a Dungeon Runners game account is created with it) the slot opens up again for you, so you can send our another invitation. The invitation will expire if it is still open after 14 days. That expiration will allow you to send out another invitation.

Additionally, current account holders no longer will need to refer themselves for that additional account. You can find full instructions on how to use the new MAKE ANOTHER ACCOUNT button in the following article: I am an existing Dungeon Runners account holder and would like to make an additional account.