Hi @slooby - there are a few ways you can handle this.
If it made sense within your environment, you could set your boards to Private and only subscribe users to the boards they should be able to see. This way they cannot navigate to other boards.
The other option would be to set these users up as Guests, set the board in question to Shareable and subscribe them to the board. You can then set the board permissions to Edit only where the user is assigned (assuming they aren’t assigned anything).
It look like @mark.anley’s post here might have answered your question, is that correct?
If so, please don’t forget to mark it as the solution so other users can easily find it, and to close this thread
If not, feel free to ask any additional questions you have!