The turn based nature of Roguelike games restricts them to handling single players only.
Why? Well consider this example: Debbie and Tom are playing NetHack together. Debbie is walking down a really long hallway, but Tom is in the middle of a battle against a Troll and a pack of kobolds, Tom will need to take much longer per turn than Debbie, since he needs to be able to select and cast spells, shift from wielding his bow to his sword, drink potions of speed, and pray for divine assistance, yet meanwhile Debbie just wants to get to the end of the hallway.
If traversing an entire hallway took just as much time as fighting a battle or casting a spell, then taking turns might be feasible. However, Roguelike games tend to have dungeon levels which are mazes of thin snaking hallways with secret doors at irregular intervals. Having to aggregate sections of hallways into large atomic chunks would deplete part of the fun of dungeon exploration. On a surface map with wide, simple open spaces it might be feasible, but not underground.
I've never been able to get a multiplayer Roguelike to run smoothly.
Mangband (which stands for Multiplayer Angband), is one of the few multiplayer Roguelike games. It can be played over the Internet (I tried it a couple of times and it was okay). Mangband takes a real-time [mud-like] approach rather than a turn based approach. It's a bit slower than traditional Angband play, and often a bit choppy due to netlag. However, if you're into competitive or cooperative play, I suggest you check it out (see my Links page).
TomeNET sounds like a cross between Mangband and TOME (both Angband variants). I haven't been able to get it to connect to a server yet.
Crossfire has been around for a few years, and has been ported to the PC, but I've never been able to run it successfully. It takes a graphical approach [using X-Windows] instead of ASCII.
There are a couple of other multiplayer Roguelikes in the works. As I check them out, I'll update this page and provide links to them.