Divide with Resource allows you to divide allocations with people that are not yet added to the project. It is located under three dots next to a project membership:
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In the upcoming dialog you can fill in:
Start date — the start date from where the allocations will be moved from
Percent or hours — How much is divided from the resourcing plan (in percentages or hours)
User — Decide who the plan is divided with (if left empty, resourcing plan will move to an unassigned role instead based on the selection below)
Role — Decide what the role will be
This would divide the membership's resourcing plan by 50% for Jack Harris who would be working with the role 'Architect' for the project