Hi Chris, and welcome!
In Trello, you should be able to invite guests to boards for free, if each client corresponds to one board. Do you have a more complicated scenario where you want to set up Client Teams or something like that, or are you ok with individually inviting clients (1 client per board)?
Here's more info around billing for multi-board guests for the Business Class Plan: https://help.trello.com/article/1123-multi-board-guests
If you had to thrive a new habit during a lockdown, what would it be? Trello
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