You can allow contacts from another project to be added to an event as guests
You just need to turn this option on for each project
How to turn the option on
Select Home in the top menu
Go to the Projects box and click Show More until you see your project
Click the cog icon next to the project
A dropdown menu appears
Select Preferences
You arrive on Project Preferences
Scroll down to the Add contacts outside the project to an event
Tick the checkbox
Scroll to the bottom of the page and press Save