This Wiki Plugin displays the number of users for a site, group, or multiple groups (since Tiki14.1), on the site.
This code:
{usercount}
Would produce on this site: 11483
By specifying the group Admins, the number of users in the Admins group will be displayed.
Admins
{USERCOUNT()}Admins{USERCOUNT}
Would produce on this site: 31
{usercount groups="Admins:Team Developers"}
Would produce on this site: 350