Very very tragic. Flooding is one of the worst natural disasters to endure it seems.My sympathy and condolences to the family and friends of the victims.
For all those wishing to volunteer searching, I would like to gently suggest that you join one of the many existing organizations AHEAD of time so you can be trained and equipped.
I lived in Kerrville for a couple years about 15 years ago, and they had a local chapter of the Red Cross and I joined for about a year and a half before I moved back to NM.
I also joined a local Search And Rescue [SAR] volunteer organization in New Mexico for around 15 years, and the local volunteer fire department for 12 years. I have not been active in either for the past 8 years or so, but will likely join SAR again now that I am retired and have the time.
It does require some training to do SAR, and and it needs to be closely coordinated by the Incident Commander [IC] so that so that you don't end up injured or lost yourself, which ends up taking away from the actual work of others searching.
Besides Samaritans Purse, there is also Southern Baptist Disaster Relief
https://www.sbdr.info/ as well as I am sure many others.