This is definitely a conundrum. To some degree, everyone should have a mind to physical defense, but there's a lot of value to relieving most people from that being their primary concern by making physical defense some people's primary concern.
There will always be some risk, especially when people who don't know each other very well are coming together to try to achieve similar goals.
Ideas:
- Rotate individuals in and out of security duties on a regular basis, like hourly. This reduces the risk that "wild cards" present, as well as helping everyone get experience doing different tasks and working with different people.
- Designate physical security roles at events which don't necessarily neeed it. The lower stakes allow room for adjustment, practice, and experience without "mistakes" creating a huge impact.