Focus groups are structured group conversations that include between 5 and 10 participants and are led by a facilitator with specific research goals in mind. Focus groups are useful if you are interested in understanding opinions about your existing product and possible features or changes that you are considering. They can also provide insight conversations that your users have when considering your problem area, the issues they encounter and the solutions that already exist. It's important to remember that people behave differently in groups, so discussions like this leave room for influence between participants and answers may not be strictly honest and accurate. With that in mind, digging into themes or opinions that arise in focus groups can teach a venture team about the potential social aspects that influence user opinions and discussions.
To conduct a focus group you need to secure a location and recruit target users. Often incentives will be needed to recruit participants. You may want to use a service or online forum to help. You will also need someone to lead the focus group who is able to present specific but open-ended questions and receive responses without feeling a need to justify or respond to comments or criticism that may come up in the course of the discussion. That person should conduct the focus group with clear research goals in mind and a spirit of curiosity about the user experience of the problem or solution. Typically focus groups are recorded so participant body language, responses and conversation can be analyzed later by the venture team.
Focus groups can be useful for discovery early on in Problem Validation but they are best suited to Solution Validation stage 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 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 and later on the discussion that you want to have with users. 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 leaders can employ when conducting a focus group and interesting angles and exercises that can be used to illicit different kinds of insights in a group setting.