The Beatitudes

Matthew 5:3 - "Blessed are the poor in spirit, for theirs is the kingdom of heaven."

Notably, Matthew uses the term 'kingdom of heaven', not 'kingdom of God'. Perhaps this is to avoid offending the sensibilities of Jewish readers - his audience was probably heavily Jewish (perhaps Jewish Christians living in 'exile' in Syria) - they might not have liked Matthew using God's name...

'Blessed', of course, is more than 'happy'; perhaps it's more like in a position of God's special favour? Does this mean the poor in spirit are in the kingdom, or that they are the kingdom, or that it belongs to them in a unique (or exclusive?) way?

'Poor in spirit' - 'those who know they're a mess'; the 'anti-Pharisees', with a humble opinion of themselves, perhaps. Just as God apparently has a special interest in the physically/materially poor, it seems He also has a special interest in the spiritually poor. Is this like coming as children (Mt 18:1-5, et al.)? Is this the tax collectors? The prostitutes?

Is the poor in spirit like a spiritual bank account thing? Like the Pharisees, who keep a close eye on their spiritual account. The poor in spirit don't have much in there, they haven't been working at earning points. Is Jesus using the Pharisaic language against them?

Is Jesus talking about in-grouping and out-grouping? Are the poor in spirit 'in'?
I suspect Jesus isn't in/out-grouping here. I think this is about the Revolution of God. Who's 'in', who's 'blessed', according to the Pharisees? They are - the 'righteous', the good Jews, the strict and particulars. In fact, some say that the Pharisees believed that their own Torah observance would precipitate the Messianic kingdom, so the poor in spirit were really messing things up as far as these guys were concerned.
But Jesus says, 'Not on my watch'. You're not cutting these guys out. This is for them too. The poor in spirit are in, they have a part in the kingdom, it's theirs. Revolutionary stuff.