@SimonErzen - I’ll provide a similar answer to the one I just used in your other post on the topic.
How can they join (what kind of settings should i have) that they can join by only having my board link?
While you can use the team invite link feature to send them a link that, once clicked, they will be on your board as an editor (if that was the permission level required), they will also be a full team member in your Free Plan. This is because the way to edit a board that resides in a Free Plan is to be a member of that Free Plan.
If you want people to be able to edit one of your boards, but do not want them to be able to see other boards in your team, they you need a paid (or educational) subscription.
You can read more about the other plans and features here:
https://help.miro.com/hc/en-us/articles/360017730233-Plans-and-features-available