That kind of happened to me after rush, too. I was dropped by all sororities during rush, and DZ was the group I wanted more than anything. The day after Bid Day, I received an email inviting me to Movie Night with the DZs. It turned out to be a COB invitation, too. I was ecstatic, and I went to the event. Two days later, I received my bid. I would've gotten it earlier, but they hadn't been able to get in touch with me!
If you're at all interested in the group, I suggest that you just reply to the email by asking some kind of question (like where you should meet them or something) to show your interest and to let them know you'll be attending. Chances are, if they know you want to come, they'll be looking for you, or they may even send someone to pick you up so you won't feel uncomfortable showing up by yourself. Good luck.
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