All adventurers start out as F-rank when they first register. The guild has a strict policy of only allowing adventurers to accept quest of their rank or lower. However a party of adventurers only needs a single member of the appropriate rank to accept a quest together as a group. For example, an F-ranked adventurer could in theory participate in a D-ranked quest so long as there is a D-ranked adventurer in the party. High ranking adventurers accepting low ranked quests is allowed, but not encouraged. Promotion to the next rank requires either satisfactory completion of multiple quests of your rank and a practical test of skills administered by a higher ranking guild member, or successful completion of a quest beyond your current rank and the recommendation of the higher ranking adventurer in your party who accepted the quest. Skipping ranks is not allowed. That is to say an F-rank who successfully completes a D-ranked quest in a party with a D-ranked adventurer could be promoted one level to E-rank, but they can not be promoted 2 levels at once up to D-rank despite having met the requirement of completing a d-rank quest.
If while fulfilling a quest you encounter challenges beyond what is appropriate for the rank of your parties highest ranked member, standard guild policy is to return to the guild and report your findings. In this event the guild will make every effort to reassign this quest to an adventurer or party of adventurers of the appropriate rank to deal with such threats. For example, If an E-ranked party is hired to exterminate mutated rats in the city sewers and during their investigation they discover the cause of the mutation is a rift connecting to the shadowfell, they should return to the guild immediately and report what they found. At that point the E-ranked quest will be considered complete and the agreed upon reward paid. A new quest of the appropriate rank will then be created to deal with the newly discovered threat.
In general, it should be assumed that an adventurer of a higher rank taking on a lower ranking quest should be able to succeed effortlessly and a lower ranking adventurer joining a party to participate in a higher ranking quest will struggle to survive.