I have a task with 7 subtasks and I would like to copy and past the group of tasks.
Maybe you could use the function "Export as template" to save it and then recover that with "Import as template". It's not as easy as copy and paste but as far as I know that function is not available.
As Alberto said, unfortunately there is no direct copy and paste feature available right now, we will consider adding this in the future. Feel free to vote on it if you'd like, http://todoist.com/Vote/
If you right click on a project and choose 'export as template' this will allow you to import the same tasks into a new project, but it does take everything from that project, not select tasks.