Hi,
You can create a custom field of the table "order" and of the type "single dropdown" via the menu Display>Custom fields of HikaShop Business to have a selector on the checkout.
The custom fields system allows you to either enter the values to be selected manually in the settings of the custom field or with a MySQL query.
You can read more about this here:
www.hikashop.com/home/blog/475-hikashop-4-4-4.html#cf
It actually allows you to easily display a select with all the users as choices of that select dropdown.
So if you want to only allow the selection of some of the users based on their user group, you could change that MySQL query to join with the user group table of joomla and add an extra condition on the user group. So it should be fairly simple if you know MySQL.
And then, if you want to be able to select users from different user groups based on the user group of the current user, you could create several custom fields, one for each user group / user (with the "access level" setting of the custom field) which will have that custom field, and in each custom field, adapt the MySQL query to filter the users based on the corresponding user group.