myGuilds.members.fetch({ user: "UserID", force: true });

myGuilds.members.fetch({ user: "UserID", force: true });

//...
client.on('messageCreate', async message => {
  if(message.content !== 'test') return;
  
  const myGuild = await client.guilds.fetch('GuildID', true);
  // "fetch" a guild
  try { await myGuild.members.fetch(message.author.id, true) } catch (e){ return message.channel.send("Author is not a member of my guild."); };
  // If message author is not a member of the guild
  try { await myGuild.members.fetch('UserID', true) } catch(e) { return message.channel.send('UserID is not a member of my guild.'); };
  // If the user is not a member of the guild
  const authorMember = await myGuild.members.fetch(message.author.id, true),
  // "fetch" message author
  authorRank = await authorMember.roles.highest.position,
  // Author's role position
  targetMember = await myGuild.members.fetch('UserID', true),
  // "fetch" the user
  targetRank = await targetMember.roles.highest.position;
  // The user's role position
  
  console.log(`Author's role position: ${authorRank}\nThe Member's role position: ${targetRank}`);
})
//...

//...
client.on('messageCreate', async message => {
  if(message.content !== 'test') return;
  
  const myGuild = await client.guilds.fetch('GuildID', true);
  // "fetch" a guild
  try { await myGuild.members.fetch(message.author.id, true) } catch (e){ return message.channel.send("Author is not a member of my guild."); };
  // If message author is not a member of the guild
  try { await myGuild.members.fetch('UserID', true) } catch(e) { return message.channel.send('UserID is not a member of my guild.'); };
  // If the user is not a member of the guild
  const authorMember = await myGuild.members.fetch(message.author.id, true),
  // "fetch" message author
  authorRank = await authorMember.roles.highest.position,
  // Author's role position
  targetMember = await myGuild.members.fetch('UserID', true),
  // "fetch" the user
  targetRank = await targetMember.roles.highest.position;
  // The user's role position
  
  console.log(`Author's role position: ${authorRank}\nThe Member's role position: ${targetRank}`);
})
//...