I took it for granted that you would come with us.
Tip: もの in this context is a natural expression with some varieties of alternatives.
- 一緒に来られるものと思っていました (original)
- 一緒に来られるものだとばっかり思っていました (っ inserted in the third pattern)
- 一緒に来れると思ってました (ら抜き言葉 + い抜き言葉)
All of them are natural translations for the English sentence.
The third one illustrates the gap between my initial assumption and the actual fact in the most surprising tone. – “Oh my Gosh! I completely assumed the scenario that you could come with us, but it turned out that my assumption was wrong”. The fourth one with っ is just a casual version of the third one.
The function of もの in this context is hard to explain, but it’s similar to the difference between “I don’t know the reason why” and “I don’t know why” in English. Inserting “the reason” is optional, but it draws more attention from the listeners.
The difference between the first 来られるものと思っていました and the fifth 来られると思っていました is like “I assumed the scenario/situation that you could…” and “I assumed you could…” in English. もの (lit. = the thing) functions as “scenario” or “situation” in this sentence structure.
The second ものだと思っていました is stronger than the first ものと思っていました due to だ, which expresses certainty of one’s belief.
The third ものだとばかり思っていました is stronger than the second ものだと思っていました because ばかり means “only” and thereby the speaker excluded other possible options from his/her assumption.
The sixth sentence is called ら抜き言葉 and い抜き言葉, respectively. They are regarded “broken” Japanese, and not recommended to use in formal writings. But native speakers often use these kinds of expressions in conversations.