Anyone with the resolve can join the adventurer's guild. Find 3 others, fill out a form, go in for an interview, pay a small fee, and you're set. There are some rules though.
The guild only accepts parties of 4. This is to minimize adventurers going missing and ensures everyone has a buddy to look out for them. Experienced parties can temporarily band together as alliances, and some even keep those alliances long term!
It also keeps things like rationing nice and simple.

*Tamed beasts and contracted entities like working fey are not considered party members.

The interview and form are to confirm your identity in the event you or items of yours go missing in the dungeon. This makes sure remains make it back to where they need to go. They also confirm your reasoning for forming the party and whether it will be temp or ongoing work.
The joining fee goes to the in-statement and management of the guild and its numerous branches, as well as other fees like medical and funerary fees. The guild provides rations and a small amount of supplies to the members as well.