Currently, resources cannot be shared between plans. As the doc explains:

"plans are completely logically separate and do not consider in any way data from other plans. This means, that the same user could be booked in parallel plans for the same time slot."

Here are some unfortunate consequences:

• JPO could not be used by an organization of any size for global resource planning.
• If one is trying to use JPO for an area with a number of independent projects and shared resources, there is no good solution:
  • If you throw everything into one big plan, you make "plan updates" problematic, since the results affect the entire plan and may need to be coordinated with many different people who are supposed to be working independently of each other - probably impractical. Furthermore, by mixing independent projects into one plan, you make the plan too "big" for comfort. (But it is true that JPO offers features like release streams and initiatives that help here. However, it looks like JPO might provide sub-plan objects like release streams to work around the problem that plans have to be bigger than they naturally would be, due to the fact that resource sharing is not possible. ).
  • If you create separate plans, you have problems allocating resources. You have to be careful to look at how a person is allocated in each plan to make sure the allocation doesn't exceed his total available hours. And you have make arrangements for him not being overbooked for a given period of time. All this without JPO support.

I think the solution is to move the "People" feature out of the plan and into a bigger object - e.g. "Organization". Here one would specify people and allocate them for given periods of time to plans. The plans would then have to work with the resource constraints handed down from the organization.