Hi Sebastian Calvo,
I have prepared a quick code example which showcases the functionality you stated.
//In your App.jsx file:

	handleAddGroupToClick(event) {
		const newItemsGroup = document.createElement('smart-tree-items-group');
		newItemsGroup.innerHTML = 'Capital letters<smart-tree-item>Α</smart-tree-item><smart-tree-item>Γ</smart-tree-item>';
		newItemsGroup.expanded = true;
		this.tree.current.addAfter(newItemsGroup, "0");
		event.target.disabled = true;
	render() {
		return (
				<Tree id="tree1" ref={this.tree} scrollMode="scrollbar" className="animation" selectionMode="oneOrMany"
					showLines dataSource={this.treeDataSource} displayMember="label" valueMember="value"></Tree>
				<button onClick={this.handleAddGroupToClick.bind(this)}> Add group</button>

Also we have demo regarding this:
Let me know if that works for you!
Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team