A charity ride is not really what I'd consider a "group ride", unless you've agreed with the people you're riding with that you're going to ride as a group. Most charity rides I've done are either just an "every man for themselves", where individual riders tend to end up by themsevles, or have sub groups of riders who know each other riding in small groups.
I'd stay away from those groups as you're not experienced with proper group riding honestly, as the riders may not appreciate you attaching yourself to their group.