When logged into your account, open the menu at top right of the story in your dashboard space and you should see the option to copy the story appear in the menu that pops up:
If you are a member of just one team that will open a window where you are asked to confirm that you want to copy the story:
If you are a member of multiple teams you will be able to you will see some slightly different options:
Here you will be able to choose which team you want to copy the story to (defaulted to the current team that story is in), and who in that team should own the copy.
If you are a member of the target team you will be able to copy it to your account (represented by You), whereas if you are not a member you can choose from a list of people who are.
If the team you are copying the story to does not have the current theme available you will be asked to select an alternative theme.
When you click copy story you will see a copied version of that story appear as the first item on your dashboard, identified with the word (Copy) added to the title:
If you have copied the story into a team you are not a member of you won't be able to see the copied stories unless you become a member of that team.
If you need to copy multiple stories at once you can use the bulk actions feature.