Learn English – Make it a compound sentence question

The solution you proposed is correct. Alternatively, if you want to omit the us/them part, you can be more creative:

My hostess asked me if I would like to attend a service in the Maori Church, to which I replied positively.

Sentence structure is totally independent of its meaning, or the thing it wants to convey. Your confusion greatly roots on the fact that you are so concerned on the actual meaning of the sentence rather than the structure itself.

But let me demonstrate why "because" falls into subordinating conjunction category: In your example

He passed because he was intelligent.

Note that there is only one line of thought here: The mere fact that he was intelligent made him pass the exam. In other words, you are basically saying that being intelligent is a cause for him passing the test.

On the other hand

He was intelligent therefore he passed the exam

Note that there are two separate states here: first, the fact that he was intelligent; and second, the fact that he passed the exam. What "therefore" did is to combine these two separate facts in order to make an inference.