If you're asking people to join a one-off focus group or discussion event, a group agreement lets everyone know where they stand and helps to create a safe space for openness and honesty. If you're setting up a regular forum, make sure you have some terms of reference in place so that expectations are clear.
To make sure you're taking everything into account when planning your activity, you might find this 'Developing your opportunity' template useful. It takes you through all the things you need to consider to promote the optimal opportunity and attract the people you want to reach.
This template enables you to ask consistent questions across all activities and throughout your organisations, the feedback people give you will help you understand people's experience of taking part and identify changes you can make to future activities.