Learn Polish: Szef proponuje im nowy projekt. - The boss is proposing a new project to them.

Im is the dative case form of both oni (masculine personal "they") and one (all other "they"). We use the dative here because the boss is proposing something to them. They are the receivers of the proposal.

Is there a longer, emphatic version of im like there is for singular pronouns (like mi vs mnie)?

No, unlike some singular pronouns that have short and long forms depending on emphasis, im is the only dative plural form for "them" when there is no preposition. You use it whether you are emphasizing the word or not!

The dictionary form of the verb is proponować. Why does it change to proponuje?

Verbs ending in -ować change their stem in the present tense. You drop the -ować and replace it with -uj- before adding the standard ending. Since "the boss" is a "he", we add the 3rd person singular ending -e, making it proponuje.

Why doesn't nowy projekt change its ending, since it's the object being proposed?

It actually is in the accusative case because it is the direct object. However, because projekt is a masculine inanimate noun (a non-living thing), its accusative form looks exactly like its dictionary (nominative) form. The adjective nowy also stays the same to match.