Ok, I was extremely surprised to meet yesterday several people who didn't figure out they could TELEPORT to Agartha (the Grid-like system in TSW) from anywhere.
It's true this is not explained. So, just in case, here it is:
You will first discover Agartha as a part of your Society first contact quest, sent there by your main contact NPC: in a warehouse room in the Illuminati HQ, in the subway station for Templar (where the guy in charge seems to be there since Queen Victoria and her husband Albert, which he both mentions), and in... a bike hangar for the Dragon.
At this point, you'll get (without any noticeable message IIRC), in your inventory, an item called "Agartha Conduit". The info pop-up for the item says "...returns you to Agartha" but the item is also tagged "Unique" and some people think it means unique usage i.e. consumed. Not at all.
Just right-click the Agartha Conduit item in your inventory and you'll be teleported into Agartha, from anywhere.
Teleporting takes a few seconds; you'll have an animation showing you hovering like in trance.
The recharge is 30 minutes; a timer is shown on the item icon.