あい（相）vs. いっしょ(一緒) 「together」
January 21, 2019
January 24, 2019
「相」（あい）is usually not used on its own, and is found in kanji compounds, like「相談」（そうだん）"consultation".
E.g. 「お姉さんに相談しました。」（おねえさん に そうだん しました。）"I consulted my sister."
On the other hand,「一緒」（いっしょ）is already a compound that can be used on its own in sentences.
E.g. 「一緒に食べませんか？」（いっしょ に たべません か？）"Why don’t we eat together?"
Hope that helps!
January 24, 2019
That provides some clarification, though now I have a new question: why is the pronunciation of 相 「あい」 instead of 「そう」 when it is paired with 乗る? (this construction is in Module 2.4)
Also, while looking around I found the verb 「乗り合わせる」(のりあわせる) which I believe means to specifically "share a vehicle"… but I don't want to complicate things further. (´∀｀；)
January 30, 2019
Without trying to make things too complicated, 「相」（あい）is the「訓読み」（くんよみ）pronunciation of the word, while the「そう」in 「相談」（そうだん）is one of the「音読み」（おんよみ）pronunciations of the word.
「音読み」（おんよみ）is the pronunciation of Chinese words in Japanese, as, originally, kanji had come from China to Japan. This is commonly found in nouns.
「訓読み」（くんよみ）is the Japanese pronunciation of words, and is commonly used in verbs and adjectives.
Apart from that, there is no easy way to tell what the pronunciation of a word will be except for knowing the word itself. Although, it might become easier to tell as you become more familiar with the different types of words and pronunciations there are : )
As for your second question, I am not sure quite what you are asking about「乗り合わせる」?
I hope that helps!
January 31, 2019
I wrote about 「乗り合わせる」(のりあわせる) because it would seem a verb tailor made for module 2.4, but after further thought あい のり します (i.e. noun+するverb) is easier to remember.