Where to present the functionality of pairing two entities in an Administration System

In this example Administration Platform, you can have users and user groups. You also have a page where you edit the users and a page that you edit the groups. And of course, you need to pair the two which can be done in two ways:

• You can have a listing of the users in the group page (or not) and be able to add a user to it.
• having and option in the user's page to add him/her to a group.

Is it a good practice to have the functionality in both the pages? Is it preferable to have more that one points where you can do stuff, or it going to cause confusion/frustration to the user?

If it should be in one of the two places, what is the common practices in one-to-many relationships, adding entity to group or group to entity?

The product is an Administration System and the user is supposed to be trained.