You can retrieve the user roles from the current user as follow:
Now, depending on your use case, you can simply retrieve the user and retrieve your desired user role and assign the user role to the user. Bear in mind that the user role has a many-to-one association to user, so if you want to add the user role to the user be sure to mark the change attribute accordingly (shown below)
If you want to retrieve current users using microflow you can use retrieve activity in which you can set the following properties.
Now if you want to set that role to any association or object you can use object-related activity in which you can add the following member.
The following microflow I used in my application to retrieve the current user and assign that user to a new incident object.
Hope this will help you.