Hi Chudanut,

The price to pay per member depends on the type of plan you would subscribe to. For instance, on a Team Plan, which is the most popular paid plan among teams up to 50 users, with the lowest price per team member, the price per team member would be 8$/month on an annual paid subscription, or 10$/month on a monthly paid subscription. You can find the price and features of each subscription on this page.

There are different ways to invite other users to your boards without adding extra paid seats in your team. For example, you can invite them as non-team users or as anonymous guest editors. On the Team Plan, you can also add users to your team as Trial Members: they will then have 7 days full access to your teams' boards and paid functionality. You can then decide to promote them to Full members or change them to non-team users (if you take no action before the end of the trial, they will automatically become Full team members).

I hope this helps!