Depending on the Admin Role and access permissions assigned to each admin, your Omnilert admins can only view and send messages to the groups to which they've been "assigned". Assigning groups can be a convenient way to unclutter the view for your senders by only showing them groups that are relevant to them.
You can review the group assignments for a specific admin account in the "Edit Admin" screens. In some cases, it may be advantageous to assign the same group to multiple admins all at once.
If you wish to quickly assign multiple admins to a single group, simply go to People >> Admins and select the desired admins in the list.
Then select Assign to group from the blue menu at the top of the list.
Then choose the desired group from the dropdown and click Add to group.
The selected admins will then be assigned to the desired group.