The Assign Contacts to Projects page lets you assign multiple contacts to multiple projects at once.

Watch the walkthrough

Steps

Go to the Contacts page
Select the More Options button
Select Assign contacts to projects
You arrive on the Bulk Assign page
There is a column for each project and a row for each contact.
Use the dropdown list to add/remove contacts to/from projects (to add a contact to a project, select Enrolled from the dropdown list)
Press Save
The assignments are done and you arrive back on the Contacts page