Preparation for a group interview is similar to preparing for individual interviews. Do you need to use a structured approach or will you aim to have a collaborative dialogue? In any case, you may need to have fewer questions than in an individual interview, since you will be hearing from more people. Consider how long your participants will be willing to spend (e.g., 1-1.5 hours) and how many questions you might be able to cover in that time.
Depending on the size of the group, it can be helpful to work with a co- researcher or colleague who can facilitate or take notes. You might record responses. If you tape record the interview, check that your tape recorder will pick up everyone's voices. You might need to borrow or rent an appropriate microphone.
As well as addressing information and consent matters, you might suggest some ground rules for the group (e.g., take turns, listen and build on each other's responses). Keep in mind that in a group interview, participants will not be unknown to each other and you will not be able make this clear when you invite people to participate.
When preparing questionnaires, keep in mind that your participants generally will read and respond to the questions, without additional explanation or prompts from you.