There's lots of ways to get around, though some are limited to certain guilds (classes), e.g. only witches can use broomsticks (with the risk of getting lost if your navigation skills aren't up to snuff), and only priests that specifically worship Hat can cast the spell that teleports you to the location of a random other player on the server (including someone who, for example, might have just been thrown into the River Ankh for failing to pay their Thieves Guild dues). I recall the most reliable method of getting around is the carriage system, where you just need to find a stop for the appropriate carriage (e.g. look at the above map for Ankh-Morpork for the Intercontinental Express carriage (if you can't find it, look for the gray circle on the central island), which will take you to any of the major cities listed in the table at https://dwwiki.mooo.com/wiki/Travel#Carriages_and_Ferries ).

As for encountering other players, a quick look tells me that there's around 50 other players online (a far cry from its heyday, but still active for a MUD in this day and age). You're likely to encounter other players spontaneously in the guild houses, but almost certainly never if you're just wandering the wilds. But there's a general chat if you just want to chill out (or at least I think there is, I haven't actually played in over a decade :P ).