Think about what you're asking ... the Government would what, mandate that you get to go to someone's BBQ? It's simply unworkable and impractical and would be impossible to draw up legislation that wouldn't impact a million and one other aspects of life.
I disagree Simon. The aim would be to prevent an organisation having an official policy of shunning, not whether an individual is allowed at another individuals BBQ.
The target is the Organisation policy and I think a government could take action, especially as it is (supposed to be) a charitable organisation.
The end result might end up the same. Brother Shunsalot still doesn't invite you to his BBQ, but Sister Shuns-cos-she-has-to would now be able to invite you round.