Some group members will be scared, so you need to cast vision for them. Let them know you believe in them and that they won't let you down. Also, follow Jesus' model when he sent out his disciples to serve. Pair them up on a task. Ask for two volunteers to plan a social event, and another couple to plan a group service or missions project.
If you've been leading the Bible study by yourself, identify one or two people with whom you could rotate leadership. Let them know you're available to consult if they have questions. They'll be terrified, but this will be the single biggest catalyst for their growth in the entire study.
Sometimes the best time to ask people to take on a service role is right after a great meeting. You close in prayer and then say, "Here's the [task]. Who will do this and where do we want to go with it? Will you take this on?"
Nine out of ten people will say yes if you ask them, especially if it's a one-time deal or for a short duration. Phrases like "test drive" and "for a few weeks" take the pressure off.
Then ask, "Who'd be willing to join Susan in doing this?" That way you have them in pairs. As people agree to roles, write down their names and their tasks. The following week, check in at the end of the meeting to find out if they've made any plans. Ask when exactly the social event or service project will transpire. People need to be reminded, so bring it up each week.