categoryID (int): This param limits the list to groups from a given category.
Example query:
https://api.countrysidebible.org/groups/?categoryID=1
This will return a list of all groups related to category 1. If you would like to include groups from more than one category, you can concatenate IDs with a + delimiter.
Example query with multiple categories:
https://api.countrysidebible.org/groups?categoryID=2+3
The full list of categories is available here: