Yes, there is! To give a co-teacher access to your account click on icon on the bottom left of your screen titled "Account"

Then click on 'Co-Teacher Settings.' Here you can invite an existing Classwork.com user by entering their email and permission level. You can click on 'Read Only' for them to just view your account, or 'Full Access' which will give them complete access to your Classwork account. You need to click on 'Full Access' to allow other teachers the ability to help with grading. 

When they accept the invitation their name and email will appear on this screen under 'Users who have access to your account.' 

If you accept access to another teacher's account their name and email will appear on this screen under 'Users who have given you access to their account.'