By default, three roles (for each kind of license) exist and cannot be deleted. These sets of actions define what can be done or not done by any user in Milliman Mind.
Create a new custom role
The first step is to click on Users & Roles on the left bar, then on Roles.
You will see the three default roles: Creator, User, Read.
To create your own click on Create a new custom role. A new window will appear.

First, give a name to your new role.
Then select in the smart selector which actions you want to allow for this role.
When you have selected one (or more) action(s) click on Add action to add it to this role's list.
You will then see the list of actions allowed with a small description.
Please also note the Minimum license level field that gives you the required minimum license level of the user according to the actions you have selected.

When your role is ready, just click on the Create new role button.
Assign a role to a user

Go to the Role section: you will be able to select in the dropdown list which role you want to assign to this user.
But do not forget that roles are related to the license level. So if your user has been granted a User-level license, you will be able to assign a User or Read level role.
Edit a role
You will be able to edit later a custom role, but: