I also agree that keeping your plans and notes to yourself is the right thing to do. I wonder if your husband is reacting to the way in which you did it. You said you were annoyed and in that situation my annoyance would've shown thru my talk with her even if I wanted to be neutral.
I suggest that because your step-daughter is your step-daughter you need to be compassionate in the way that you talk with her. Tell her that you understand that she wants to be an important part of the group and you can see how doing what she did make her feel good. Then explain, still in a loving voice, how in the long run she would've felt the other girl's anger because it would appear that she's more important than they are. Tell her she's an important part of the group and that you are responsible for being sure that she is treated that way. Give her a hug, if she'll let you. Tell her you love her.
She's wanting reassurance that she is important and special. Focus on her good qualities. Praise those qualities. I assume you praise all of the girls in the group. Be sure to also praise your step-daughter. Share a special something with each girl. For example each week one girl can have a special job. Rotate among all of the girls, including your daugher.
I also like the idea of making something for the other girls, such as a hair ribbon or cookies as suggested by Jody P.