Here's a possible solution, given in the form of an employees-to-projects m:m relationship. Each employee can work on many projects, each project can involve many employees.
From left to right, you show the following:
A panel showing the details of the currently selected employee.
A list of all employees, where each item in the list shows the employee's name as a clickable link or button (to display details in the detail panel). At the head of the list is a toggle button which filters the projects list to only those associated with the currently selected employee. At the foot of the list is a button to add a new employee, which display an empty details panel ready to accept input.
A vertical space in the middle with a single "Link" button allowing the user to link the currently selected employee with the currently selected project. Clicking this button would open a dialog allowing the user to enter details of the link (i.e. how long the employee is assigned, what role the eployee will play, etc).
A list of all projects, where each item in the list shows the project's name as a clickable link or button (to display details in the detail panel). At the head of the list is a toggle button which filters the employees list to only those associated with the currently selected projet. At the foot of the list is a button to add a new project, which display an empty details panel ready to accept input.
A panel showing the details of the currently selected project.
Obviously, you'd have to limit the size of the details panels, maybe by only showing the details relevant to the m:m relationship. You might even add a button on the details panel to open a more detailed pop-up window, or you might do away with the details panel altogether, if you're mainly interested in managing the links. This UI would work really well on wide-aspect screens.
HTH! Klay