The help on this topic is:
shared_folder_id optional The ID of a specific shared folder.
include_membership Required if shared_folder_id is specified. If true, include a list of members and a list of groups for the shared folder.
show_unmounted optional This value, either true or false(default), determines whether the returned list of shared folders will include shared folders that the user has left (but may still rejoin).
My interpretation is that include_membership=True MUST have a shared folder ID. Ideally I'd like to have include_membership WITHOUT a shared folder ID and get a JSON return with all the shared folders and all of the members of the share (basically in one REST request).
The other option, which I'm doing now, is to get the shared folder metadata and for every folder call shared folder metadata again supplying a shared folder id (retrieved from the original get shared folder metadata request) with include_membership =True.
Thanks,
kevin