When it comes to the meeting itself, don’t overdo it. Get the right people around the table. This means the person signing off on the software, the vendor (who will answer any questions), and yourself. If you won't be responsible for implementation, consider whether the person who will should be present, particularly if they'll have a view on how the new solution fits your existing systems.
In smaller practices, three people around the table is often enough. In larger firms, you may also want your practice manager or IT lead present if they'll own the rollout.
Anyone else can crowd the meeting, so be sparing with any additions. What you want is a clear decision by the end.