<name> + imnida. = I'm <name>. Sarahimnida. / 새라입니다. = I'm Sarah. They're saying "Kuresenimnida" because they think your name should be spelt 커레센 / ku-re-sen. Just spell the name close to how it sounds in Korean. Sarah / 새라 / sae-ra. That's how I understand it, at least.

What SarahCC said about the -imnida (-입니다) suffix is correct - it's the most formal form of the verb "to be" and it gets attached to whatever noun is its subject. My guess is that your friends are saying that the name Cresen would be spelled 크레센 in Korean. (The way Sarah spelled it - 커레센 - is not a conventional transcription and would be pronounced differently).