Final answer:
The correct fill-in-the-blank answers are 'their' and 'there', with 'their' indicating possession and 'there' denoting location.
Step-by-step explanation:
The correct words to complete the sentences are 'Janelle, Marcus, and their group went to the library to prepare for the presentation, but the specific book they needed wasn't there.'
The key is to understand the correct usage of homophones in the sentence. 'Their' is a possessive pronoun that shows ownership, indicating that the group belongs to Janelle and Marcus. 'There' refers to a place, indicating the location of the book. As such, 'they're' (a contraction for 'they are') would not fit contextually in either blank.