Demonstrates how to add UserFieldValue for a Group as value in ListItem via SharePoint JSOM API

addFieldUserValue.js

function addListItem(list, properties) {
    let ctx = list.get_context();
    let itemCreateInfo = new SP.ListItemCreationInformation();
    let listItem = list.addItem(itemCreateInfo);
    for (let name in properties) {
        listItem.set_item(name, properties[name])
    }
    listItem.update();
    return executeQuery(ctx);
}

function executeQuery(context) {
    return new Promise((resolve, reject) => {
        context.executeQueryAsync(function () {
            resolve();
        }, function (sender, args) {
            reject(args);
        });
    });
}



let ctx = SP.ClientContext.get_current();   
let group = ctx.get_web().get_associatedMemberGroup();
ctx.load(group);
executeQuery(ctx)
.then(() => {
    
    let groupVal = new SP.FieldUserValue();
    groupVal.set_lookupId(group.get_id());

    let list = ctx.get_web().get_lists().getByTitle("Tasks");
    let taskProps = {
       "Title": "Task123",
       "AssignedTo": groupVal
    }
    return addListItem(list,taskProps);
})
.then(() => {
    console.log("Saved");
})
.catch((sender,args) => {
   console.log(args.get_message());    
});