what is meant by “The first group (of people) who will enter Paradise, will be glittering like the full moon, and those who will follow them, will be glittering like the most brilliant star in the sky.” [Al-Bukhaari and Muslim],,,, how many groups will enter paradise ?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Allaah The Almighty told us (in the Quran) that the prosperous people will be driven to Paradise in groups just as the wretched people will be driven to Hellfire in groups, Thus, the prosperous people will enter Paradise one group after another. Allaah The Almighty did not tell us the number of those groups, and this is a matter of the unseen that one cannot speak about except from divine revelation (i.e. Quran or Sunnah).

However, the Prophet, sallallaahu ‘alayhi wa sallam, told us the description of some of those groups. He, sallallaahu ‘alayhi wa sallam, told us, as in the Hadeeth which you stated  in your question, that the first group to enter Paradise will be (glittering) like the full moon. The prominent scholar Al-Qaari argued that they are the Prophets, may Allaah exalt their mention, and then they will be followed by a group of those who are next to them in virtue which are the Awliyaa’ (close friends of Allaah), scholars, martyrs and the righteous. They will look like the most brilliant star in the sky (i.e. in brightness).” [Summarized from Mirqaat Al-Mafaateeh]

Allaah Knows best.