Don’t capitalize the first word of a tag that comes after dialogue; it’s not the beginning of a new sentence. Also, always use some form of punctuation at the end of the dialogue, whether there’s a tag following it or not.
“You’re one of them now,” she whispered softly.
Softly is not needed after whispered, because a whisper is soft. On the other hand, if the adverb were harshly, for example, or delightedly, that would add meaning that isn’t conveyed by the verb alone.