Yes, you can. Assign the colleague a task, @mention them in a note, or assign them an email attached to the work item, and they will get access to that work item but not the private contact. If you want them to automatically have access to all future work for this client, you will need to add them to the client team.

So long as the client team for the private client excludes them, they will only be able to see the work item because they are now a member of the work team.

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