Can I refuse a grant position I was offered and still hope to work there in future?
Professionally, they should understand, if explained to them in a polite and respectful way. After all it is your life, and they need to respect your decision. Yes, it is disappointing, but it should not burn bridges - good people are not easy to find and they should be happy to keep in touch; who knows what the future brings.

However, if they were so disappointed that they would be indeed offended by your decision, then it is likely that there are other problems lying in wait for you at that group, anyway.