Tips for collecting onsite social media content from my crew?

This is definitely one of those thing you have to have constant messaging with your team about. You need to talk to them about the WHY behind it. How it benefits them, feature them with tags if they like that, and they need to see you post often so they know it's going somewhere. Then keep talking about it. Every other weekly meeting, when you see them on the job site. If it's something you are always mentioning, they will understand the importance of it.