Whats the best way to tell my office about my engagement?

I recently got engaged. We have already told our parents and some of our siblings. This weekend we will finish telling our siblings and can finally announce it to everyone (I'm talking Facebook official!). I work in a small law firm (I'm a file clerk) where we mainly do divorces. It's an all female firm, most of which are unmarried, so not surprisingly they don't all have the best outlook on marriage. However, we all care about eachother and get along well, so I know they will be happy for me. But how do I tell them? Making a big announcement would be uncomfortable for me, yet an office email seems too impersonal! Any suggestions?