No, you don't need to add markers to a layergroup, nor do you need any plugins. To add a marker to the map you can simply do:
var marker = L.marker([50.5, 30.5]);
marker.addTo(map);
If you need to move the marker programatically you can use setLatLng
:
marker.setLatLng([40.5, 40.5]);
How you decide to map users to markers is really up to you. A simple solution could be to use an object where keys are userId's (or some other user identifier) and values are markers:
var users = {
11: L.marker([35.5, 15.5]),
15: L.marker([40.5, 20.5]),
17: L.marker([45.5, 25.5])
}
Have you read the Leaflet documentation?
http://leafletjs.com/reference.html
You can also read this short example on using Leaflet on mobile devices: