The definition of a cross-site
<www.microsoft.com/.../technolo
gies/sharepoint/wssadmin/stsglos.asp?frame=true#C> group says:

A custom security group that applies to more than one Web site. A
cross-site group can be assigned to a site group as if it was a single
user.



However this article
<msdn.microsoft.com/.../spptsd
k/html/tsovsecurityusersgroups.asp> says that you can also assign
cross-site groups to site groups in a site collection.



Currently as far as I know the only way of grouping sites is by creating
sub-sites within a site (thus forming a site collection).



Isn't there a way of grouping several top-level sites, because currently
if a site collection can only be formed by having subsites, isn't the
flexibility reduced?



But on a side-note it is possible to create cross-site groups at the
main portal level just by creating them at the following url

http://server/_layouts/1033/mygrps.aspx