Get an idea of projected usage so that everyone feels they have equal access.
See if you like them
Make sure they are all solvent and can put their hands in their pockets when needed.
Check them out with other people you know - there are a lot of 'grey-listed' pilots out there you would not want to fly with.
Get hold of a set of draft syndicate rules - esp rules relating to getting out of the syndicate. Leaving a group is much more important than joining one, and much more difficult and emotionally drainiing if you get it wrong.