You could split using a regular expression /(?<=\)),\s/:

const str = "1. Crowe, Velvet (LoC), 2. Hume, Eleanor (Ext), 4. Shigure, Rokurou (DmN), 10. Mayvin, Magilou (MgC)";
const res = str.split(/(?<=\)),\s/);
console.log(res);

This will split on ), as suggested by cmgchess in the comments. The issue with splitting based on a string is that it removes some of the characters you want to keep. Instead, by using a regular expression, you can use (?<=\)) (here ?<= is called a positive lookbehind) to match and keep the ) in the resulting split element. The ,\s then splits based on a comma followed by a space (\s).

You could also use the following regex with .match() which is a little more robust, but may need to be updated depending on the text that can come after your number/rank:

/\d+\.\s\w+,\s\w+\s\(\w+\)/g

The above performs:

• \d+\.: Will match one or more (+) digits followed by a dot
• \s: Match a whitespace character
• \w+,: Match one of more (+) word-characters (\w) followed by a comma ,
• \s: Matches a space
• \(\w+\): Match word characters surrounded by parentheses ( )
• /g: Global flag to match the sequences for all occurrences in the string

const str = "1. Crowe, Velvet (LoC), 2. Hume, Eleanor (Ext), 4. Shigure, Rokurou (DmN), 10. Mayvin, Magilou (MgC)";
const res = str.match(/\d+\.\s\w+,\s\w+\s\(\w+\)/g);
console.log(res);

Just a little improvement of @cmgchess's answer (add trim method to remove unnecessary spaces):

const getUsers = (str) => {
    const users = str.split('),').map(x => x.trim() + ')');
    users[users.length - 1] = users[users.length - 1].replace('))', ')');

    return users;
}

console.log(getUsers("1. Crowe, Velvet (LoC), 2. Hume, Eleanor (Ext), 4. Shigure, Rokurou (DmN), 10. Mayvin, Magilou (MgC)"));