If we set ACL for a user's calendar to another user, we can use something like this :
sogo-tool manage-acl add user1 'Calendar/personal' 'user2' '["PublicModifier", "ConfidentialDAndTViewer", "PrivateDAndTViewer", "ObjectCreator", "ObjectEraser"]'

Result :
ACL is OK

But if we set ACL for a user's calendar to a group, we should use something like (seen this from sogo-tool manage-acl get ...) :
sogo-tool manage-acl add user1 'Calendar/personal' '@my_group' '["PublicModifier", "ConfidentialDAndTViewer", "PrivateDAndTViewer", "ObjectCreator", "ObjectEraser"]'

Result :
ACL is set for a user with name 'my_group' but not for the group with name 'my_group'
If we go to webmail, the ACL entry is display like if the groupname is a user.