No, it will not harm your individual credit. Even after marriage your individual accounts will still only show up on your credit (and hers on her credit, etc.).
But until her credit is cleaned up, you could have a very difficult time getting joint credit together, particularly mortgages. Working as a mortgage banker, I see this issue all the time. One borrower will have strong credit while the other will have poor credit. If the borrower with the strong credit also has the majority of the income or the ability to qualify on his/her own, then usually we will do the loan with just that borrower, leaving the other borrower off (but they have to agree to that or we won't do it). If the borrower with the strong credit cannot qualify on their own, and/or the poor credit borrower is the one with the income, then things may get interesting and we might have to turn the file down.
edit: just to clarify, in general, most lenders require that either (a) both borrowers qualify credit-wise individually, meaning both borrowers must have sufficient credit strength to qualify, or (b) they qualify based on lowest middle credit score of both borrowers (for credit score driven programs, like 2nd mortgages, HELOCs, and alternative and subprime loans).
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