Hi @fabijay88!
This will take a small bit of scripting to create, and you have two options to accomplish this.
Get roles by user
- get all of your users. /api/v2/users
- run each user_id against this endpoint to get respective roles /api/v2/users/{id}/roles
OR
Get users by roles
- get all of your roles /api/v2/roles
- run each rule_id against this endpoint to get respective users /api/v2/roles/{id}/users
Let me know if this helps you 
.