There are two types of ownership for a Project: individual and group. There must be an individual Project owner, but a Project does not have the have a group owner. Perform the following steps to change the ownership for a Project.
First, within the project, click on the "Team" tab (this might look different if you have the alternate Project UI):
Then, choose the "Edit Team" button in the upper-right corner:
Finally, scroll to the bottom of the members list and click on the small "Edit" button in the lower-right corner:
You should see the following:
You can now choose individual and group ownership from this pop-up. Make sure to click "save owner"! If you click outside the box before saving your changes, your changes may not take effect.
Note: If you would like to remove group ownership altogether, choose "No group" in the Group owner list. You can still invite QUBES members into the Project even if the Project is not affiliated with a Group.