If I want to express that someone has been to see me, for an informal visit, do I say she has been to see me or she came to see me? Is there any difference in meaning?
Both are perfectly correct.
Which one you prefer depends on the context.
We are more likely to use has been for a recent visit and came for an earlier one.
If the visit took place that day, you are more likely to say:
She's (just) been to see me
without a time reference (although people might add this morning/afternoon)
But you can't say she's been to see me last week.
If the visit had taken place some days/weeks/months earlier, you are more likely to say:
She came to see me (before Christmas)
either with or without a time reference, depending on the context.