Я бы дал go member.displayName
Вот пример, который я использую
bot.on('guildMemberAdd', member => {
// ADD MEMBER ROLE ON JOIN, this is optional
member.addRole(member.guild.roles.find(role => role.name === "Member"))
const channel = member.guild.channels.find(ch => ch.name === '?arrivals')
const welcomeEmbed = new Discord.RichEmbed()
.setTitle(`New member has arrived`)
.setColor(0x8A3F3F)
.setDescription(`Welcome to server, ${member.displayName}!`)
channel.send({ embed: welcomeEmbed })
})
Так что в вашем случае вы можете просто получить избавьтесь от const channel и замените channel.send на member.send