Adding new engineers to an existing project or team can be a challenge for many reasons.
If your team is new and growing rapidly (startup), it might be hard just to find time to work on on-boarding process, docs, and trainings. If your team is well-established, there could be a ton of 'tribal knowledge' about how and why things are the way they are. And if the new hire is replacing someone who's already left, well, there can be knowledge gaps.
As a hiring manager I've seen it go both ways. I'm curious what has worked well for you, either when joining a new team or adding people to your project.