Regular dungeons can be found all over Madrigal. Everyone can join these dungeon as long as you meet the level requirements. You will find other players in these dungeons even if they are not in your party. This means that also the boss of these dungeons are available to everyone.
Instance dungeons are only accessible if you are in a party. These dungeons are party bound, so you will not find someone in the dungeon that does not belong to your party. To enter the dungeons you need to be in a party but also meet the level requirements.