Adds labels to gmail threads if members are in contacts group

update-emails.gs

// Add Gmail labels by contact group
// komarov.vitaliy@gmail.com, 2018

// Inspired by https://github.com/thirschbuechler/GmailContactGroupLabels
// BTW: see readme in mentioned github repo ^^^

function applyGroupLabels() {
    var MY_GROUP_NAME = "My-contact-group-name"; // Change right side of this line to match your contact group
    var MY_LABEL_NAME = "My-label-name"; // This as well, insert the Gmail label
    var QUERY = 'in:all -in:trash -in:spam'; // This as well, insert the query
    
    var labeledThreads = [];
    
    var emailAddresses = getEmailAddressesFromContactsGroup(MY_GROUP_NAME);
    var threads = getAllThreadsByQuery(QUERY);

    threads.forEach(function (thread) {
        // Main logic here:
        emailAddresses.forEach(function (emailAddress) {
            var messages = thread.getMessages();

            messages.forEach(function (message) {
                if (message.getTo().indexOf(emailAddress) > -1) {
                    labeledThreads.push(thread);
                } else if (message.getFrom().indexOf(emailAddress) > -1) {
                    labeledThreads.push(thread);
                }
            });
        });
    });

    var label = GmailApp.getUserLabelByName(MY_LABEL_NAME);
    label.addToThreads(labeledThreads);
}

function getEmailAddressesFromContactsGroup(groupName) {
    var emailAddresses = [];
    var groupContacts = ContactsApp.getContactGroup(groupName).getContacts();

    // Collect all email addresses from contact group
    groupContacts.forEach(function (contact) {
        var contactEmailAddresses = contact.getEmails();

        contactEmailAddresses.forEach(function (emailAddress) {
            emailAddresses.push(emailAddress.getAddress());
        });
    });
    
    return emailAddresses;
}

function getAllThreadsByQuery(query) {
    var threadLength = 1;
    var i = 0;
    var allThreads = [];

    var threads;
    var start;
    var step;

    while (threadLength) {
        if (i === 0) {
            threads = GmailApp.search(query);
            threadLength = step = threads.length;
            allThreads.push.apply(allThreads, threads);
        } else {
            start = step * i;
            threads = GmailApp.search(query, start, step);
            threadLength = threads.length;
            allThreads.push.apply(allThreads, threads);
        }
        i++;
    }

    return allThreads;
}