What does Lady Macbeth mean when she says 'That which hath made them drunk hath made me bold'? Who are they?

Lady Macbeth is referring here to the guards who protect
King Duncan. Part of the fiendish plot to kill the king in order for her husband,
Macbeth to succeed to the throne involved drugging the  king's guards. It is Lady
Macbeth who conceives the plan to ‘drug their [the guards]
possets.’

 The whole household of Macbeth has been
celebrating the arrival of King Duncan. His visit was to give thanks to Macbeth for his
bravery in battle in defeating the Norwegian forces. Lady Macbeth has convinced Macbeth
that killing the king will advance the prophesy told by the witches that he should be
king. Macbeth is not so sure of the plan, but Lady Macbeth convinces him of the
necessity to kill the king.

As the evening progresses she
becomes more determined in her plan. The adrenaline (and alcohol) which has fuelled the
evening has given her courage to carry out the plan, hence
–

That which
hath made them [the guards – and possibly the other guests] drunk hath made me
bold.