Regular, but not too frequent, communication with all those associated with the group is an excellent way to let people know what's going on, and to make everyone feel included. Usually this is done by e-mail, if the organisation's policies allow it. It could also be done by text if the group was small, although the amount of information that can be sent by text is very small.
To achieve maximum effect, newsletters should be kept as short as possible and made as visually attractive as possible. They could, for example, list details of the meetings of the group over the next week or so, air future plans and ask for comment.
It should be remembered though, that those who are on the fringe of the group are not likely to be motivated to come to group meetings by e-mail alone. E-mails lack the personal touch; it is always necessary to back up e-mails by speaking individually to people from time to time and to make them feel included as valued members of the group.