Error

Error #0338: you can't add a user group ID (XXX) to the not auto-created conference UID (YYY)
 

Error description

A conference, where you attempts to add a group of users, is not auto-created one. User groups are can only be added to auto-created conferences (Eliza the bot is the owner of those conferences, UIN=0).

 
Problem solution

Make sure that you choose a correct conference UID. Usually this issue occurs when you work with MyChat Server directly by using an official open protocol.