CCP Games is now going to provide every player with a buddy link that can be used to invite more friends at once. Once someone uses the reference link to upgrade his trial account to the full version, the referenced player will gain rewards.
The invitational link will not expire after activation. Instead that link can be passed on to as many people as possible and anyone and everyone can use it to join the EVE universe.
The direct personal invites will still be available for players to use.
For more information you can check out the official blog post.