If you are using Laserfiche user accounts (as opposed to Windows accounts), you can create a database view that gets a list of all Laserfiche users in a particular group, such as the Managers group. Below is an example:
CREATE VIEW [dbo].[managers]
AS
SELECT *
FROM
(SELECT trustee.trustee_id, trustee.trustee_name FROM trustee) t1
INNER JOIN
(SELECT * FROM grouplist WHERE group_id = 'X') t2
ON t1.trustee_id = t2.member_id
GO
Replace the X in group_id with your own. You can find the group_id in the dbo.trustees table. It's the trustee_id of the row where the trustee_name is the group name.
Once you create the view, you can hook it up to your dropdown field using a Lookup Rule.
This approach works very well because your dropdown list will automatically get updated as you add/remove managers to/from the Laserfiche group in your repository.