Think about a realistic number of people to make the group workable. It’s unlikely that you’ll recruit large numbers to a group at first and in fact it can be very difficult to make sure that everyone can have their voice heard in large groups. On the other hand, if you only have two or three people you may not be able to reach a consensus and as they are volunteers (who may also be very ill) you cannot assume that all members will attend all meetings.
Around six people is probably a good number to aim for. But, don’t take too long on recruitment as you risk alienating the people you have already signed up if they are waiting around for meetings to begin.
As above, once you have a group set up it is vital that you continue to communicate with them so that they know what is happening and continue to feel involved in your work.