Once you've clearly defined your group and made a commitment to lead it, the next question is, "Can I handle it alone?" If you have a choice, answer "No!"
A co-leader can either serve as an assistant to you or as one who almost equally shares the load. How you decide this will depend on your own skills, interests, and available time. The key is to be sure that you've communicated clearly and that both of you agree on what each will do.
I'm a firm believer in co-leaders or assistant leaders, for several reasons. A co-leader can provide you with a perspective that alone you'll probably miss. Leading a group can take a lot of attention to stay on track. A co-leader can pick up on needs through comments or body language that may fly right by you.
A co-leader is essential if you have a mixed group. A co-leader can monitor your timing, signaling you when it's time to cut off the study and move on to prayer requests and prayer. Sometimes when I'm in the midst of a good study I forget all about the time. I need a good co-leader to wave her arms at me and insist that we simply must stop. A co-leader can fill in the awkward gaps or rephrase a question when necessary. This is especially important early in the life of a group when people may still be reluctant to answer. The co-leader can also help you set the pace for openness and vulnerability.
A co-leader can relieve you of much of the responsibility for follow-up and other personal contact with members of the group. Note that I said much, not all. You'll still need to make some phone calls and stay in touch. But your co-leader can take the bulk of that responsibility.
Your co-leader can take over if you're sick, out of town, or can't attend for any other reason. Your group will flounder ...