In this case, you can create a custom role for this project that includes all permissions Team Members have + a permission to prioritize. To do so you need to:
1. Open the project settings in the old (silverlight) version: http://gyazo.com/25c70329c461022ae86821a3194b9595,
2. Go to Permission Groups and create a new group with the desired permissions: http://gyazo.com/4c9943ff490205322c6146536f075c52
3. Then change her role to the custom one you just created: http://gyazo.com/a6bb42056fc9ff80fa96e2b1df2a5614
Do not hesitate to contact us again if you need any further assistance.