Sometimes particular specialism is needed on a project that your team doesn't have. This could be test automation, devops, or a particularly tricky integration. Instead of telling the client to create a separate project for another team, you may also proxy for the specialists. Here's how that works:
Talk to your teammates and agree that you want to bring in a specialist
Talk to client and explain what's going to happen
From your team page, go to the hired guns and click 'Hire gun'
Talk to the hired gun to make sure they're on board with the client expectations
A hired gun can book just as any team member, but only on that project. Hired guns don't get to manage your team. Often your hired guns are also team members in another team. Take extra care to manage calendars and expectations well.