The answer to your question is based on the opportunity cost of the different opportunities you already have. Finding a suitable job as a new employee is typically very tough in the construction sector. Even if you want to work for free on a construction site, there are several challenges. The businesses would add you to their payroll, which would make for an excellent learning atmosphere but not a great career one. Therefore, I would suggest joining NICMAR if you're having problems obtaining a job that's decent.
However, if you have a job, I suggest you continue with your job and gather the learnings and later pursue a PGP in management from a better institution.