I like 'conventicle'.
I imagine them standing round in a circle so I might say:
'a round of wizards'
'a ring of wizards'
or
back to the Celtic theme and Druids who were often sages, shamans (or shamen :-) and mystics maybe:
'an order of wizards'
... o.k., o.k. .... maybe not
Come to think of it, were/are wizards not for the most part Einzelgaenger?