Below is some basic information on who to contact if you would like to support clubs. It is in no way exhaustive, but gives you a good indication of where to start.
STEM Clubs Network
The STEM Club Network is managed by STEMNET. There are a number of ways you can contact STEMNET about STEM Clubs:
To work with clubs in your region contact your local STEMNET contract holder.
To volunteer, consider becoming a STEM Ambassador. You will receive training and a CRB check, as well as being notified of different STEM volunteering opportunities. Details are here, and your STEMNET Contract holder will also be able to help you with this.
Do you have an activity you think would be suitable for STEM clubs? If your activity is local then contact your local STEMNET contract holder and for national projects contact email@example.com. Also consider using the National STEM Centre elibrary or the STEM Directories to share details of your activity.