Disadvantages include the need for extra planning time to prepare together so that you work together in synchrony. Use group leadership techniques. These involve structuring, linking, summarizing, asking questions, and dealing with disruptive members. Provide plenty of snacks. Snacks before the group, during breaks, or even food as a focal point, having a group in conjunction with a dinner, can provide a great social atmosphere that fosters group cohesion.
Good food is usually a great incentive.
Let everyone take some responsibility. Make sure the dads own the program. The program needs to be owned by the fathers themselves. Make sure it is the men's program and build the agenda around their needs. But don't expect everybody to show up every time.