Unfortunately it's not possible to create this kind of lookup using the out-of-the-box customization tools.
There are some unsupported ways to do this using JavaScript, however if you want to do it in a supported way your best option is to simply create 2 many-to-many relationships; 1 with user and 1 with contact. Those can then be displayed on your form in sub-grids
still allowing you to select a combination of many contacts and many users.
Hope that helps
Paul
If my response helped you find your answer please show your thanks by taking the time to "Mark As Answer" and "Vote As Helpful".
