Everyone is able to invite others to the clan in-game via clicking on a player's name in the lobby, and click Invite to Clan. People can accept the invite by going to Barracks and clicking Clan Invites. There have been some issues with this working properly, and the fixes usually end up restarting the game, or just giving it time and checking back on it occasionally.

In order for people to be invited to the clan, they must also create a CoD Account, which can also be accessed via their Barracks. It will say COD Account at the bottom, and make sure they verify their account by going to their e-mail. This may speed up the process by which they can accept the clan invite.

Thanks,
-refraKtion