Currently, there are already shared locations in RTM. However only the person who created those shared locations is able to pick those shared location upon initial creation of a task. When you created a task for a shared list and you want to link it to a location that is not created by you, you have to follow these steps:
1. Create task
2. Save task
3. Edit the task and select the shared location
4. Save the task again.

Idea:
Make shared locations really shared, so that you can pick any of those locations upon creation of a task.