Guest just mean that they are only a member on that specific board (opposed to being a member of your entire workspace and can see all your boards).
On the specific board they have the same permissions/can do the same as normal members.
If you need people that can see, but not edit they need to be "Observers" (which is a Premium Paid plan or high feature)
(A bonus of keeping someone as a guest in paid accounts is that they are free if they are only on a single board)
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