Learn English – What does “What have you been up to lately?” mean

It is "have been". It is basically never correct to say "am been", and it wouldn't mean what you want it to mean anyway.

One possible, correct sentence is:

I have been, for a short time, an employee of that company.

Or (this is more natural for me, but arguably less correct):

I have, for a short time, been an employee of that company.

Either way, it means the same. It says that you were employed a short time ago. It doesn't mean you're about to leave. I see why you might want "from" in this case, and not "for", but don't panic: in this case, "for" does not mean that it will only be "for a short time"! Just that it is "a short time" so far.

Now, you said you didn't want a grammatical or technical explanation, so we can stop there. But in case anyone else wants to know the grammar behind it, I'd like to elaborate a bit anyway.

A construction like "have been" is called the present perfect. It is for actions that are past and finished, at this moment. Its structure is have + past participle.

(There is also a past perfect, which is for actions that were already past and finished at some previous time. Its structure is had + past participle.)

A construction like "am being" is called the present continuous. It is for actions that are happening (right now), or that often happen (not necessarily right now, but before now and, we expect, after now too). Its structure is be + present participle.

But "to be" leads a complex existence. It is also used in ways that are basically unrelated to the present continuous.

A construction like "am been" is called the present passive. It is for actions where the subject ("I", in "I am been") is the recipient or target of the action. Its structure is be + past participle. You might say "I am insulted" (someone gave you an insult), or "I am seen" (someone saw you), or "I am blessed" (someone gave you a blessing).

It would be very unusual to say "I am been", though. That would mean someone was being you. If an English speaker really wanted to say that, they would almost certainly use different words.

You've got the meaning of the sentence reasonably accurate - they made an offer, you made a counter-offer, they rejected your counter-offer. They're politely telling you they'll keep your information, but that may just be due to legal requirements. In all likelihood, it will end up in the circular file (a euphemism for the garbage can).

In either case, what you should do from this point is not really a question for ELL, but rather The Workplace.