virtualbox ubuntu下使用设置

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　　VirtualBox is not currently allowed to access USB devices. You can change this by adding your user to the 'vboxusers' group. Please see the user manual for a more detailed explanation.
　　
　　http://www.cyberciti.biz/faq/howto-linux-add-user-to-group/
　　

Howto: Linux Add User To Group
by Vivek Gite on March 14, 2006 · 85 comments

　　How can I add a user to a group under Linux operating system?
  
You can use the useradd or usermod commands to add a user to a group.  The useradd command creates a new user or update default new user  information. The usermod command modifies a user account i.e. it is  useful to add user to existing group. There are two types of group.  First is primary user group and other is secondary group. All user  account related information is stored in /etc/passwd, /etc/shadow and /etc/group files to store user information.
useradd Example - Add A New User To Secondary Group
　　You  need to the useradd command to add new users to existing group (or  create a new group and then add user). If group does not exist, create  it. The syntax is as follows:
useradd -G {group-name} username
In this example, create a new user called vivek and add it to group  called developers. First login as a root user (make sure group  developers exists), enter:
# grep developers /etc/group
Output:
developers:x:1124:　　If you do not see any output then you need to add group developers using groupadd command:
# groupadd developers
Next, add a user called vivek to group developers:
# useradd -G developers vivek
Setup password for user vivek:
# passwd vivek
Ensure that user added properly to group developers:
# id vivekOutput:
uid=1122(vivek) gid=1125(vivek) groups=1125(vivek),1124(developers)　　Please  note that capital G (-G) option add user to a  list of supplementary  groups. Each group is separated from the next by a  comma,  with  no  intervening  whitespace. For example, add user jerry to groups admins,  ftp, www, and developers, enter:
# useradd -G admins,ftp,www,developers jerry
useradd example - Add a new user to primary group
　　To add a user tony to group developers use following command:
# useradd -g developers tony
# id tony
Sample outputs:
uid=1123(tony) gid=1124(developers) groups=1124(developers)　　Please  note that small -g option add user to initial login group (primary  group).  The group  name  must exist. A group number must refer to an  already existing group.
usermod example - Add a existing user to existing group
　　Add  existing user tony to ftp supplementary/secondary group with usermod  command using -a option ~ i.e. add the user to the supplemental  group(s). Use only with -G option :
# usermod -a -G ftp tony
Change existing user tony primary group to www:
# usermod -g www tony