Is this gramatically correct? It is Randy and her whom they want to see.
It is almost correct. I believe it would be, "It is Randy and she whom they want to see." When a pronoun follows a linking verb, such as "is," the pronoun should be in the nominative case, such as he, she, we, or they. At least you got the who versus whom correct!
The difference between "who" and "whom" is always confusing for people. A little trick to check in the future is to turn what you are trying to say into a question and then answering it yourself using either "he" or "him" for "who" or "whom" respectively.
Who/Whom do they want to see?
Choice 1: They want to see he.
Choice 2: They want to see him.
In this case, him is what you would pick, so the question would be "Whom do they want to see?" and you would answer, "It is Randy and she whom they want to see." If you're ever still in doubt, you could always rewrite the sentence so the issue doesn't even arise. For instance, you could just say, "They want to see Randy and her," or say, "It is Randy and she they want to see."
Finally, depending on what this sentence is for, it might be better to keep it as "Randy and her" even if that's not correct. "It is she" is a very formal way of speaking, as most people don't think about how "she" is the nominative form or anything like that. Therefore, if you are dealing with dialogue or another place where it wouldn't make a lot of sense to be completely grammatically correct, you might want to keep it as you have it.