You could run communications 05 like office linked but I would recommend 07. Do you have server 00, 03 or 08? 90 users would still fall under the standard edition with 1 server but I would recommend 2 servers for anything over 100-150 users (in case you have to think about expansion). Redundancy, lag and uptime all play a factor
I also asked about a price range, what is your budget?