This feature is currently in Early Access. If you're interested, please contact your customer success manager to check if your organization is eligible.
Assign a global role
Click your organization name on the lower-left corner of the page and select Users.
Click on the user's right-hand side and select Change global role.
In the pop-up window, select the global role from the dropdown menu.
Click Update.

Assign a group or site role
When assigning roles from a role page, users who haven’t been added as a member to the site or group will automatically be added.
Limitations
Group and site roles require a full seat. You can't assign one to a user on a lite or guest seat.
One role per group or site. A user can hold a different role in each group or site, but only one role within any single group or site.
Scoped roles depend on your site visibility setting. For group and site roles to work as intended, your organization's site selection settings must be set to "Users can only view and select sites they're a member of".