A(A short comment first: I'd use the generic "People" rather than "The people.")
The general rule says that adverbials should come after objects, so a more correct sentence would be:
- People did not exploit the resources well.
"well" is quite a light adverb and perhaps a better one should be found for your sentence to sound more idiomatic, as well as semantically richer. It also does not sound nice at the end of the sentence, after a comparatively longer object like "the resources". However, note that the mentioned problem disappears with a pronoun as direct object: "People did not exploit them well."
I suggest revising your sentence as follows:
- People did not exploit the resources effectively / successfully / properly.
The rule also says that (usually one-word) adverbs of manner can also be placed between the subject and the verb and in the middle of verb forms consisting of more than one word, and that they usually have to be placed in mid position in the passive voice:
- They quickly exploited their advantage. (simple tense)
- The resources were not well exploited. (passive voice)
- The architect has cleverly exploited new materials. (perfect tense)
- The company is widely exploiting its recent discovery. (continuous tense)
Unlike longer, more meaningful adverbs like the ones above, "well" can only be found in mid position in active (non-passive) sentences when it takes an intensifier, e.g. You very well know who is to blame.
It also appears as forming part of compound adjectives like "well-known".