Next to the task you want to share to another project, click the details arrow.

When the details bar opens, scroll to Projects. The projects where the task is currently located will appear along with the section they are located in. To add to another project, select "Add to projects".

Select the project you would like to add the task to. 

*Keep in mind: This does NOT create a duplicate task. The task will exist simultaneously in all of those projects.  If you complete the task in one project, it will be completed in all projects it is associated with.

Close the details window. You will now see that task in any of the projects you have added it to. 