I know that "between" is used when the individual items are specified and "among" is used when they are not. Ref:
https://www.grammarly.com/blog/between-among/ . Examples:
- I need to choose between Yale, Oxford and Stanford.
- I need to choose among these universities.
Based on that, "among us" should be correct and "between us" incorrect, but I am not sure this is always valid. For instance:
- Let's keep it between us.
- Let's keep it among us.
Both sound correct to me. Is that right? If so, is it an exception or have I misunderstood the rule used to choose between "between" and "among" ?