Copy-paste is limited at the moment. You can copy and paste text within text fields, for example while you're editing Comments or Activities.
It is currently not possible to copy and paste Dreamler elements like Groups, Activities and Milestones. Many features like copy-paste appear simple to implement, but are rather complex in a multi-player real-time environment where actions are based on previous actions. We are working on this challenging feature and plan to release it in the near future.