If you are organizing onsite sessions, check with the trainer ahead of time as to what equipment and connectivity is necessary, and be sure the room is appropriate. If training is outside the office, be sure everybody receives pertinent information such as directions, office hours, room location, transportation, etc. Let the trainer know beforehand about special issues with the software so they will be included as part of the agenda. Get manuals ahead of time, and encourage staff to review them. That sometimes is a cost issue.
Some trainers charge a fee based on the number of attendees, others charge a flat hourly fee.
Classroom-style group training can work if the trainer can keep everyone engaged. But this can be difficult, given the wide range of computer experience of the users. Busy attorneys may not be amenable to long, group training sessions that, by definition, move at the speed of the slowest student. Some trainers separate support staff from attorneys just because of different personality and role issues. In any case, designate at least one individual as the firm trainer.