I have an donated organ, will a streptococcal b infection cause serious complication?

Immune status? If you are on immune suppressive therapy after your transplant, any significant infection can cause serious complications. I would encourage you to speak with your transplant team, even if it has been some time since the transplant, or an infectious disease doctor, both of which can give good advise about your current immune status and risks from specific infections. Remember to get your flu shot!
It can. This bacteria is not usually invasive for people that are not receiving immunosuppressive medications, are not pregnant or elderly or have poorly controlled diabetes. It colonizes the gut and the perineal area. With reduced immunity it can invade the blood stream and caused serious infection. The good news is that it is still very sensitive to plain penicillin.