Pager Genius has been helping people page their clients at trade shows since 2008. Here is how it works: you will have a greeter at the booth, and the greeter will write down the potential clients' names and the pager number assigned to them and quote the approximate wait time.
When an appointment window opens, the greeter will page the prospective client and wait for them to respond. The client can be up to a mile away and know it is their turn to be seen by staff. Pager Genius does offer rental paging systems for trade show events.