Welcome to the SR family!
You have found a wonderful resource of information and support. Please make yourself at home by reading and posting as much as needed.
I recommend spending time reading in the permanent posts at the top of this main page. Those posts are referred to as the stickies. The stickies contain some of our stories, and loads of wisdom from members sharing their personal experience.
I think your oldest daughter is ready to hear some of the truth about dad and his friends. I believe they need to understand that dad is going through some changes and he is getting support from a group of men that have more experience and are willing to help their dad make positive changes.
I hope you will share with both children that the changes in dad are not something they have caused. I still remind my children that they did not cause the alcoholism in our family and that they did not cause the separation of our family (my AH chose not to quit). Children internalize things they don't understand and begin to believe that they caused changes in their loved ones ~ when they are not responsible for the changes.
If your children are in school, you may want to talk with the school counselor about ways to address this with words they will understand.
Have you attended Al-anon meetings for your own face-to-face support? I found the meetings to be of great benefit in my own personal recovery from living with active alcoholism. I even took two of my children to an open Al-anon meeting as a way of introducing them to the meetings and introducing them to the option of attending Al-ateen meetings if they wanted. For now, they are choosing not to attend Al-ateen meetings.
Let us know how we can help support you during this time.