The phrase "recruiting sergeant" refers to a person who tries to convince other people to join a particular organization or cause, typically by using persuasive tactics. Full definition
«Guantanamo Bay, which was an international abuse of human rights, acted as recruiting sergeant for dissidents and alienated Muslims and alienated many other people across the world,» said Mr Hain, speaking on BBC1's Sunday AM programme. (politics.co.uk)
Wishart saves himself by pointing at the PM and calling him «the best recruiting sergeant for the SNP». (politics.co.uk)
The Ukip leader is planning to say: «This person is an excellent recruiting sergeant for Ukip. (theguardian.com)