There are plenty of daily dungeon runs, you just need to use the default LookingForGroup channel:
/join LookingForGroup
or as an alternative the world one
/join World
I suggest you to install an addon like this one:
https://www.curseforge.com/wow/addons/lfg-group-finder-bulletin-board
Also this is the latest sums up about Era and specifically the activity on Firemaw too.
https://eu.forums.blizzard.com/en/wow/t/wow-classic-era-update-2022-wrap-up
I don’t know why you can’t find guilds in-game but here are listed all the raiding ones:
https://www.reddit.com/r/classicwow/comments/vy59yd/undead_era_the_actual_state_of_the_classic_era/
…and there are plenty ones of those focused on levelling too not listed in there that you can find in-game and on the Vanilla Classic Era discord here:
https://discord.gg/ZVVPRt8YGK