Can I only select tracks that belong to a specific group? in Pro tools

You should be able to use something like this:

function getGroup(name) {
    var groupList = sf.ui.proTools.mainWindow.tables.whoseTitle.contains('Group').first;
    return groupList.childrenByRole("AXRow").allItems.map(function (r) {
        return {
            row: r,
            selectBtn: r.childrenByRole("AXCell").first.children.first,
            name: r.childrenByRole("AXCell").allItems[1].children.first.title.value.match(/^.+ - ([^(]+)/)[1].replace(/ $/, ''),
        };
    }).filter(function (r) { return r.name == name })[0];
}

function selectGroup(name) {
    var grp = getGroup(name);
    if (!grp) { log('Error', 'Group "' + name + '" not found'); throw 0; }
    grp.selectBtn.mouseClickElement({});
}

selectGroup("Group 1");