If you send a group message on your iPhone, anyone else who uses iMessage will respond to the entire group, but that is not the case in your situation.
If one or more recipients is using standard SMS (on an Android, etc.), their response will be a standard SMS message back to you.
You can not prevent getting single messages as replies as long the group is not on same system.
In your case you can create a group in your contacts, then use it to reply to all (from the single message you got).
Here are more details from Apple how to.
You can send a message to two or more people at a time using SMS, MMS,
and iMessage. You can send messages to multiple people with group
messaging and have any responses delivered to everyone in the group.