I'm using this in the login script.

if member ".group_name_cn.all_groups_ou.main_ou.my_org.my_tr ee" then
map root m:=\\one_group\data\group_name_folder

I have a container for all my groups that is at the same level as the
container for my users. I put the above statement in my users container so I
can map a drive only to members of "group_name_cn"; why won't it work.

I have replaced "if member" with "if group" and it will map if you are part
of the group, but if you are not part of the group, you will receive errors.
"if member" produces no errors nor does it map the drives if you are part of
the group.