If you have Active Directory and run XWiki on a Windows server, you can use the Waffle Java libraries to allow single sign-on (SSO) using Windows integrated authentication to XWiki. See the following:
In this configuration you set the xwiki.authentication.ldap.group_mapping setting in xwiki.cfg to something like this:
xwiki.authentication.ldap.group_mapping=\
XWiki.XWikiAdminGroup=CN=XWiki Admins,OU=Groups,OU=DC=fabrikam,DC=local|\
XWiki.MarketingUsers=CN=Marketing,OU=Groups,DC=fabrikam,DC=local|\
...
We use a configuration like this in our organization and it works very nicely.