How to move paragraph styles into a group?

Try the following

(function () {
    var doc;
    try { doc = app.activeDocument; } catch (e) { alert("Open a document first."); return; }

    var targetGroupName = "AA";
    var styleNames = ["a1", "a2"];

    // Get or create top-level paragraph style group by name
    function getOrCreateGroup(name) {
        var grp = doc.paragraphStyleGroups.itemByName(name);
        if (grp.isValid) return grp;
        return doc.paragraphStyleGroups.add({ name: name });
    }

    // Move style into group (or duplicate if move not allowed)
    function placeStyleInGroup(styleName, group) {
        var style = null;
		for(var i = 0; i < doc.allParagraphStyles.length; i++) {
			if(doc.allParagraphStyles[i].name == styleName) {
				style = doc.allParagraphStyles[i];
				break;
			}
		}

        if (style && style.isValid) {
            if (style.parent.id === group.id) return;
            try {
                style.move(LocationOptions.AT_END, group);
            } catch (e) {
                alert("Could not move style")
            }
        } else {
            // Create under group
            try {
                group.paragraphStyles.add({ name: styleName });
            } catch (e) {
                // fallback: create at doc level then move
                try {
                    var ns = doc.paragraphStyles.add({ name: styleName });
                    ns.move(LocationOptions.AT_END, group);
                } catch (ee) { alert("Failed to create style '" + styleName + "': " + ee.message); }
            }
        }
    }

    var targetGroup = getOrCreateGroup(targetGroupName);
    for (var i = 0; i < styleNames.length; i++) placeStyleInGroup(styleNames[i], targetGroup);

    alert("Done.");
})();