Discord.js | How to get members currently connected to a Voice Channel?

Your bot needs GUILD_VOICE_STATES intent

const client = new Client({ 
  intents: [
    Intents.FLAGS.GUILDS,
    Intents.FLAGS.GUILD_MESSAGES, 
    Intents.FLAGS.GUILD_PRESENCES,
    Intents.FLAGS.GUILD_MEMBERS,
    Intents.FLAGS.GUILD_VOICE_STATES
  ] 
})