It should give you a little more detail in the rejection email you received. Most likely you were declined for having the same IP address as another user that already plays. Do you know anyone who uses the same internet connection as you do?
The join form includes a field in which to enter the handle of someone who needs to be registered as a multi. It's her own fault for not entering your handle in that field, and until she does so, she'll likely continue to be declined.