Since this question does come up from time to time I will answer it here.
The procedure is described in the forum Guidelines which is linked to at the top of the main forum page, and the information on group forums is at the bottom of the Forums Guidelines page.
What it says there is:
Group Forums
To request a Group Forum, first make sure your SL Group has at least 20 members, then send an email to [email]groups@secondlife.com[/email] with your SL name, group name and the name of 2 group members who will be moderating the group forum. Please note, Group Forums which go unused may be deleted.
To request a Group Forum, first make sure your SL Group has at least 20 members, then send an email to [email]groups@secondlife.com[/email] with your SL name, group name and the name of 2 group members who will be moderating the group forum. Please note, Group Forums which go unused may be deleted.