Learn Dutch: Nodigen jullie de collega's uit? - Are you inviting the colleagues?

It is part of the verb uitnodigen (to invite). This is a separable verb. In a normal main clause or a question, the verb splits: the core part (nodigen) goes near the beginning, and the prefix (uit) gets pushed all the way to the end.

Why does collega's have an apostrophe? Isn't that for possession?

In English it is, but in Dutch, an apostrophe plus s is often used to make plurals! You do this for nouns ending in a single open vowel (like a, o, u, i, or y) to keep the vowel sound long. If you just wrote collegas, the Dutch pronunciation rules would make the a sound short.

Why does the sentence start with the verb Nodigen?

Because this is a yes/no question. Dutch doesn't use helper words like "Are" or "Do" to make questions. Instead, you just swap the subject and the verb. So jullie nodigen (you invite) becomes Nodigen jullie (are you inviting).

What is the difference between jullie and other words for "you"?

Jullie is strictly the plural "you" (like "y'all" or "you guys"). If you were asking just one person, you would use the singular jij and say Nodig jij de collega's uit?