Welcome to the Community, it's great to have you here!
If I was in your situation, I would suggest that you create a free team (unless you already have Business Class) and add the individual clients to the individual boards that you want them to have access to.
Free teams are limited to 10 boards, however, there is currently no limit on the number of free teams you can have.
So if you have 3 clients with 5 boards each, it might be worth creating 3 teams, one for each client.
However, I would caution you to not add them at the team level, only add them to the boards you want them to be able to see. If you add them at the Team level - they will have admin rights for the whole team, including upgrading to Business Class, which could be very expensive for you (not them).
Let me know if this is helpful! If not, we'll attempt something else!
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Trello...
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