Being that the Brightside is a basic phone, it might not be capable of responding to a group. The option does seem to be there, though - pg. 48 of the user manual:
While viewing a message touch to display available options:
• Reply w. Copy: Reply to the sender, plus other recipients if desired,
and include a copy of the original message.
Using the "More" option, you can apparently add recipients, or reply to more than one.