Yes, in principle, all mortals can join :-) - Cardano is permission-less. However, mortal or immortal, at the moment only people who held ADA in a Daedalus or Yoroi wallet on November 29th can participate in the current test phase. In a few months, staking will come to the Cardano main-net at which point everyone will be able to delegate. I will be glad if you join my pool :-)