Yesterday I activated the character slot unlock on the character select screen.
There are 2 ways to unlock additional characters. Firstly, you simply play the game. As you level your characters you are awarded IP Points. These points tot up on your account, and enable you to unlock additional characters.
The second option is for those that would like to support Project Terran, a character slot unlock is available in the TSC Store, in the Account Upgrades section of the store.
There is a new button on the character select screen, over to the right on the blue section, like this:
If you have enough IP, you will see a green button. If the button is red, hover over it and it will tell you why it is not available.