You can put vias down quickly by pressing P, V, then Tab to set the via properties. When you place it down, it will attempt to guess what net to take (or ask you).
To my knowledge, Altium doesn't have a tool that will autplace vias like some more RF oriented packages have. If you need a big grid of them though, you can lay a few down to get your spacing, then start copy-pasting the vias.