Trello, does free version support single board guests?

[Werner Beck]

We're just starting to use Trello, as a non-profit organization.

Two questions have come up so far:

1. does the free version support single-board guests?
   If so, how do you set these up?
2. should we (have to) decide for the paid version, what's needed to receive the non-profit price/rebate?

Thanks
Werner