First of all have a good group -- something that doesn't already exist on dA, a group other artists need, a group which offers them something other groups don't -- either because the other groups are inactive or because other groups don't fit the specific needs this new group is going to fulfill.
Then you invite people who might need the group.
Then let it run its own course. If the group is truly useful, it will attract new members on its own. No need to excessively promote it.
If you want the group to grow larger than it already is, good promotion tools are
> contests (only for group members
~ and make membership easy -- just a click of a button -- auto approve all memberships )
> interactive journals -- blogs that stimulate discussion or art creation
> adding artwork or journals (of non members) to the group -- more people will learn about your group in a positive way, because you use your group to showcase another artost's work.
> affiliating with other groups
> adding the group icon to your comment signature -- that way every time you comment, people will see your group -- (an icon draws more attention than a link)
> having an attractive looking group icon -- make the icon visually appealing.
^^ at least these things seem to work
just my observation so far