Bots in a group chat only receive messages directly addressed to them.
Type @, and then select the bot from the list of group participants. Or type @ immediately followed by the bot name [i.e. @fraserthebot]. Type your message, and then wait for the bot to respond.
Also, based on the Skype developer's documentation:
In a group the bot will only receive messages directly addressed to it e.g. “@fraserthebot This is the message”. It will not receive other messages sent by group participants or notifications of users joining or leaving the group.