Environments can be grouped for organizational purposes. If an environment and an individual user are in the same group, users will be tagged with inherited on the Manage access page. This means that the user is inheriting their access from the group, not the environment.
If you manually assign a user to an environment, and they are already assigned to it via a group, they will be tagged with override on the Manage access page, indicating that their individual access will override that of the group for this one environment. You can then modify their access in this special case.
From the menu select Environments. Locate the environment you want to give users access to then select Manage access at the end of the row.
Next, select the users or teams you want to add using the dropdown. Then use the Role dropdown to select the role you want this user or team to have.