A mastermind is only as good as the mix of people in each pod. You are selling access to peers as much as access to you, so selection is the most important thing you do. Do it with care and the software barely matters.
Use a short application. Ask what the applicant is working on, what they want from the group, what they can offer others and how much time they can commit. The answers show you who will contribute and who will only listen. A polite no to someone who would not fit protects the members who would.
Match members into pods on stage and situation, not on enthusiasm. People at a similar point in their business have problems the others recognise, and that is what makes the advice useful. If you mix very different stages, the newer member learns a lot and the more experienced one leaves.
Run a short kick-off session for each new pod. Cover the format of a meeting, the confidentiality rule, how commitments are posted and what happens if someone misses sessions. Write the same points in the pod space so members can find them later.
Plan the end of a pod as carefully as the start. At the close, ask each member what they got, whether they want to continue and whether anyone wants to move pods. Use the answers to reform pods, and move members who have outgrown a group to one that fits.