Make it clear to project champions how to invite new people to Operately

[markoa]

"As a project champion I'm trying to include everyone from my team as project contributors so I can ask them to provide weekly status updates in Operately. But I cannot find them in the list of available people to add as contributors, nor invite them to join. Which makes me feel frustrated that I have to break focus from setting up the project until I figure out how to add them." — Alex from Semaphore