There is nothing about "joining" a militia. A basic translation into today's English of the 2nd Amendment would sound something like this:
Because we may find it necessary to call up and train a militia of citizens, the citizens' right to own and bear arms shall not be compromised.
Actually ignoring that they had recently fought a war against their own government which had turned to tyranny seems to be a regular infantile fantasy of the "left"... The idea that you should want to ban guns because you chose a blue jersey over red is just idiocy. Stop falling for everything people say and start thinking. That goes double for my side, stop promoting nonsense because it is a platform plank...
Nobody believes in every plank of a platform, that somebody who swore to uphold the constitution would promote this inane "only if you are in a militia" crap is beyond me. Your oath did not have an expiration date.