I’m obtaining the users on the system with the GET /user API call. I’d need to get also the groups that each user belongs to, but I only see a GET /identity/groups call, that requires a user id. Is there a way to get this data without calling the mentioned method for each user?
What is your use case for this? It seems odd that you need this information for every user.
Generally, you will have to correlate the 2 data sets somehow. There are multiple options for this, e.g. a custom SQL query.
If you only want to use the provided REST endpoints, it might result in less requests if you query for all groups, and then fetch all members of each group with the User Query and memberOfGroup Parameter.