there is no ADD button and I am unable to remove selected users from the group
Where exactly is the button not displayed? Could you, please, post here or send us (support[at]adaxes.com) a screenshot?
Are the Security Role settings correct or is something missing?
The Security Roles look correct to grant group owners the permissions to modify all properties of the groups they manage.
Once the SRs are correct, what needs to be set in the web interface config for the necessary options to become available?
Could you, please, provide us with all the possible details about the options not being available? Please, post here or send us (support[at]adaxes.com) screenshots of the pages where you expect the options to be displayed.