In FileMaker, this is called a "conditional value list". Here's a Knowledge Base article to get you started. If that's insufficient, post back and we'll take it further: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list/session/L3NpZC83enlCdXc4aw%3D%3D
This has two things it needs to do.
1. Match "qualifications". These must be the same in both Task and People.
2. Eliminate people already assigned.
#2 is more difficult. This is known as "dwindling". It needs to assemble a list from all people assigned to ANY task, so it can then omit them. One way is to use an "X" (any, Cartesian) operator, via a self-relationship in Tasks. Another would be to use a Value List of the same, and use ValueListItems (Get(FileName), "value list name") to get them. It may be faster, but you'd need to hard-code the name; which I try to avoid.
To omit them you use "≠" (does not equal) as the operator. But that needs at least one value to work, so a dummy value is used if the above list is empty.
It also requires a bit of a "kick upside the head" to refresh the value list. So a script trigger runs to Commit Record, then Refresh [ flush cache join results ].
Because the Value List may have no values after you choose the person (if there's no one else with matching skills left), a regular "straight" relationship to a table occurrence of People is used to show the name. You'd want it anyway. The choice is a drop-down rather than a popup showing only the name. The drop-down is hidden behind the arrow graphic.
Thank you. I get this. The example, is actually simpler that what I need to do, but it gets me pointed in the right direction for working on it this weekend. Your the best.