Instructions
Open the Roblox website from within the Roblox Studio application.
Click the places tab and click the "Configure" link for your place. This opens the "Configure Place" web page. Scroll down to the "Copy Protection" section and uncheck the "Copy Lock My Place" box. Click the "Update" button. This unlocks your place so that others can copy it.
Message your friend and tell him to open Roblox Studio and navigate to your Roblox place on the website. Tell him to click the "Edit" button under the picture of your place. This stores a copy of your place on his computer and opens it in his Roblox Studio. Once he is done editing tell him to save it and choose "Publish" to load it as one of his places. His place also needs the "Copy Lock My Place" setting unchecked.
Navigate to your friend's place on the Roblox website and click the "Edit" button. Save the place, choose "Publish" and replace your old place with the new version. Your place now includes your friend's modifications.