When an admin manually adds a new subscriber using the People >> Subscribers >> Add Subscriber utility, those new subscribers will be created as "Inactive" subscribers by default. (This allows you to create a number of new subscribers ahead of time, then activate them when ready.)
How to "Activate" an inactive subscriber
To activate their accounts, simply visit the People >> Subscribers >> Inactive page. Then select the desired subscribers and choose "Activate" from the menu at the top of the list.
How to manually create new subscribers as "Active" subscribers
If you wish to have new subscribers be immediately "active" in Omnilert simply choose the "Add subscriber and make active" option when creating their new account using the dropdown right on the Add Subscriber button on the People >> Subscribers >> Add Subscriber page.
This option adds the new subscriber to the "active subscribers" list immediately upon the creation of their new subscriber account.