Setting up a portal for external access is something you will need to involve you network team with to setup the right infrastructure and authentication. In AX2012 R3 there was support added to help with alternative authentication methods to allow vendors to log on. The roles you assign to those users will likely be similar to this setup for test/demo purposes.

This video was recorded with AX2012R2 but the same concepts apply to AX2012 R3.

Cheers

Lachlan