Does this Czech sentence mean: (1) who is next to me or (2) who supports/agrees with me, or (3) both?
Most likely (2). But (1) could be used in some cases, but some additional pronoun would be there, like "Kdo je tu/to se mnou?" = "Who is there with me?"
I think (1) can work fine also without a pronoun.
It just needs a proper context which is not hard to imagine.
(2) can definitely work without a pronoun and is not too hard to imagine. igamenir only wanted the pronoun for (1).
Sorry. I was talking about (1). I have corrected the previous post. So, can it be for (1) meaning without a pronoun?
Theoretically yes, it is grammatically possible. "Kdo je s tebou?" is very common.