This is a failure of government, rests at Trump’s desk, that he has not placed enough troops at the border to secure it. Civilian militias should be in the way, then go home. Until the government can properly secure the border—now with officers and troops, later with a wall, then, yes, civilians should volunteer and help. Maybe the government might embed them to ensure safe operation?
This monologue from Tucker explains the reason of the problem you are talking about much better than I could. 9 minutes, excellent.