For the situation you described, I would not use a pop-up dialog, but rather a mater-details pane (see below). To add a new node, the user would click the tree on the branch she would like to add the item to, and then click on the "Add" button (didn't understand why you used right-click when you have a gorgeous "Add" button there).

The details of the node (in this case a new user) would be edited through the form on the right. I know it demands a lot of space, but it's a lot more comfortable than using the intrusive pop-up. You might want to consider also adding Save/Cancel buttons to the details form.

In our system we have lots of tree, and many boast CRUD operations on them. Sometimes, when there is no room, the new node details would be inserted through a pop-up dialog, but I prefer this happening right next to the tree when possible. This also side-steps the loading time issue of the pop-up. Personally, I hate pop-ups because they are intrusive and modal.

^{download bmml source – Wireframes created with Balsamiq Mockups}

For the situation you described, I would not use a pop-up dialog, but rather a mater-details pane (see below). To add a new node, the user would click the tree on the branch she would like to add the item to, and then click on the "Add" button (didn't understand why you used right-click when you have a gorgeous "Add" button there).

The details of the node (in this case a new user) would be edited through the form on the right. I know it demands a lot of space, but it's a lot more comfortable than using the intrusive pop-up. You might want to consider also adding Save/Cancel buttons to the details form.

In our system we have lots of tree, and many boast CRUD operations on them. Sometimes, when there is no room, the new node details would be inserted through a pop-up dialog, but I prefer this happening right next to the tree when possible. This also side-steps the loading time issue of the pop-up. Personally, I hate pop-ups because they are intrusive and modal.

^{download bmml source – Wireframes created with Balsamiq Mockups}