thanks everyone, i was finally able to join a dungeon with another player.
the precedure is as you guys had described...form a party, buy a tablet and enter via the stone. I think the initial problem I had was forming an actual party as I can't read any japanese so it's hard to navigate. there's quite alot of friendly players online, and I was able to catch up with a few US players who showed me the basics.
I think only one person in the party has to use a tablet, the other party member clicks on the stone and follows the other person directly, without having to enter any prompt at the stone.
from what ive seen the silver tablets take you to a stronger area of the dungeon where the enemys are more agressive and the dungeon itself is larger. their use is once only. from what I've been told, the ash tablet has infinite use.