Hi,
The "Command Lookup Tool" is your friend for this, here's the reference:
http://www.cisco.com/cgi-bin/Support/Cmdlookup/home.pl
And here's the output:
The aaa server-group feature introduces a way to group existing server hosts. The feature enables you to select a subset of the configured server hosts and use them for a particular service.
A server group is a list of server hosts of a particular type. Currently supported server host types are RADIUS server hosts and TACACS+ server hosts. A server group is used in conjunction with a global server host list. The server group lists the IP addresses of the selected server hosts.
Examples:
The following example shows the configuration of an AAA group server named tacgroup1 that comprises three member servers:
aaa group server tacacs+ tacgroup1
server 10.1.1.1
server 10.2.2.2
server 10.3.3.3
HTH
Andrew.