If it's booked in advance it's not an open mike. It's a short
gig. Soon they'll all be booked in advance, then further and
further in advance.

Your friend should set it up such that there's a lottery
for two-song or ten-minute (whichever comes first) slot a half
hour or so prior to each event.

Those who are present (not their buddy, not their girl- or boy-friend...the principal, and not out in the parking lot
or in the can, or 'on my way'; people at the open-mike stage
of their career may as well learn about playing by the rules
and being on time) get a numbered ticket.

As numbers are pulled out of a hat, they get to put their
name in for their preferred slot in the rotation (some people
don't like to be first, others hate being last). If there
are gaps, the emcee reserves the right to make adjustments
though...it's just a preference.

As a courtesy, those who are in the first half of the evening
are expected to stay and listen until the break. Those in the
last half are expected to be there for all of that half. Those
who are too self-important to do so lose their slots and future
invitations to participate.