Usually I just load like 5 or 6 rounds into a mag and tell someone to take a shot or two in semi then switch it over. That way if they start to walk the gun its running out right away anyway. Burst mode is a good alternative.
As long as people are friendly I'll let them shoot my stuff, most don't even know you can own full auto and I want more people in the mindset that we should be able too. Its worth saying too I mostly shoot at private outdoor ranges where I've found that as a general rule we are dealing with more responsible gun owners than the idiots at the public indoor ranges.