how to add external users?

Hi, I have a sharepoint 2013 site and I got the users from an active directory, now I have to add external users but I can't add these to the active directory, how can I do it? I have imagined two ways:

1. create a local active directory in my machine, add users and import them from central administration (I don't know if this is possible)

2. add form web authentication and create the users in it.

there is any other way? what do you recommend?

luis molina