If you want someone to be on the list of People who can be assigned to work on a client job or opportunity, they need to be marked as Available in Resourcing. Similarly, if they are not marked so, they won't be able to be assigned to work on client jobs or opportunities.
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Go to the Person's Profile by navigating to the People List (Zones > People), and then click into the Person's Profile.
Once there, go into Edit mode (top right of the page), and scroll down to the Organisation and Resourcing section.
Tick (or untick) the box to make someone available/unavailable in resourcing, and make sure you click Save before leaving the page.
TIP - If you're not marked as Available in Resourcing, you won't appear in the default People list in Zones > People. To read more about this, click here.