Focus groups can be useful for discovery early on but they are best suited to Solution Validation and later as a forum for testing new features or changes to an existing product.

Focus groups are useful when you want to gain insight into the opinions of your users in a group setting. Having a specific concept, prototype, brand or experience to research is important if you want to get actionable meaningful insights.

Focus groups can be very formal or more conversational but it's important that they be structured and in a controlled space like a closed office or meeting room. Be sure to prepare a plan to guide your recruiting the discussion that you want to have. Questions can come in lots of different formats to tease out different insights. The Interaction Design Foundation has a comprehensive guide here that outlines tactics that facilitators can employ when conducting a focus group and interesting angles and exercises that can be used to elicit different kinds of insights in a group setting.