Possible that other people joining Miro board dont have access to all of your boards in free plan?

​@SimonErzen - The only way to edit a board that resides in a Free Plan is to be a member of that Free Plan. And, because the Free Plan does not support private boards, i.e., setting the team access to "No access", all Free Plan members can see all boards in the team (they can also create unlimited boards in the team).

And the only way to stop other Free Plan team members from editing other boards is to set the board's Team permissions to Viewer or Commenter. 

is it possible that other people joining my Miro board dont have access afterwards to all other boards of yours?

After the session, you would have to delete the users from the team, and disable the team invite link that you used to invite the other participants (if you used that feature).