As someone who is a member of two clubs (one a walking where we do lunch after a group walk, and the other an automobile club that meets up once a month for lunch). In both cases, there are generally 15-20 people or more having lunch. In both cases, the restaurant changes each time. For the walking club, upon arriving we let them know that there will be separate checks. For the automobile club, there is an advance reservation. In either case, as the server takes orders, we let them know who is together.
Definitely let the server know when he/she arrives that there will be separate checks and as initial orders are being taken, let them know who is together. It will expedite the delivery of the checks at the end of the meal.