Thanks to Justin Lai for the inspiration for this challenge.
Click the blue "Share" button on the top tool bar and type in the email addresses associated with your teammate's Onshape accounts. Make it public so that anyone with the link can view what you've made.
You can scale any object with the "transform" tool in the Parts Studio
Right click on a part from the menu on the left and select "Edit in Context". You can then transform or modify the part as you would in a Part Studio while seeing how it fits in with the assembly.
From Onshape's home page, you can search all public documents from the top bar (make sure you are looking in "Public", not "My Onshape"). When you find a document you like, you can make a copy, then right click the part you're interested in and move it to the document you and your partner are working on.
If your parts aren't all combined into one piece like you intend, you can select them all and use the "Boolean" tool to make sure they're all one part.