I cannot understand in this sentence the various functions of the components.
I understood the meaning ("Germans are not used to be surrounded by a lot of people", non-literally) but I can't well understand the structure.
Up to 他们旁边 it's fine: "their side". Now there's 围, and I can't understand if it's here as a verb or as another noun. In this last case, it would be determined by 他们旁边, and we could maybe translate it as "their immediate surroundings", since 旁边 gives me the idea of "very close". At this point though, 满 must be the verb, but Pleco says that, as a verb, you can use it only to express the meaning of "to fill" (unlike 充满， that actually would work better since it expresses "be filled with"). But if this is the case, its object should be the object that it fills, like a cup. 人 has no sense as an object in this case.
Maybe 满 is a complement for 围? Can be used as a complement? And why the use of 了?