The Atlassian Community can help you and your team get more value out of Atlassian products and practices.
I'm a new nonprofit and I'm using trello for our team projects. Do I have to pay for every team member who want's to collaborate?
You can add outside Guests to single boards in order to collaborate on your team boards. It is not required that all members are on a team. If Guests are added to a single board within the team, the guest is free of charge.
To be transparent though, if you add users outside your team to more than one board in your team, the Multi Board Charge will apply: Multi-Board Guests. Guests that are just on a single board in the team are free of charge.More details about how billing is working with Business Class is available here: How Billing Works with Trello Business Class.