In The Force Awakens Maz Kanata says
I am no Jedi, but I know the Force.
Are there any other groups who have learned to use the Force, or is every non Jedi/Sith force sensitive completely untrained?
Between canon and Legends, there are plenty of Force user organizations. Here's a list:
From current Disney canon, the Dathomir Nightsisters or witches were featured in the Clone Wars cartoon series, which is considered current official canon.
There are the Knights of Ren.
The Knights of Ren was a dark side organization that operated under the command of Snoke, the Supreme Leader of the First Order. Kylo Ren was a member of the organization.
The Knights of Ren operated in the decades following the Galactic Civil War. They were under the command of Supreme Leader Snoke, who used them as a tool of the dark side of the Force alongside the First Order.