Sync2Social is a free app that lets share with your Facebook Groups. It’s free and simple. Here’s their description:
Sync2Social: Syncing items to your specified social groups efficiently with just simple clicks!
Sync2Social is the first social app to enable you upload items to your specified social groups (such as Facebook groups or Facebook Wall) efficiently with just simple clicks using your mobile phone; As well as download messages from your social groups, so you see what people talking about within that group.
In Sync2Social version 1.0, you can take photo, select picture or write a text using your Windows Phone 7.5 (Mango), and then post them to your specified Facebook groups or Facebook Wall. You can also see and reply messages in your social groups
You can check it out here for free.