Hi Jane,
We've begun using a tiered service model based on
Cornell's to deal with capacity constraints and also because some people just want different levels of service. In the "Consultant" role, we limit our time commitment to a maximum of 4 hours -- that generally looks like one chunk of time to get them going and another chunk to review the search strategy.
We've also had to ask people to wait a couple months if they want us to co-author. We have a 5-person team, so sometimes things can be handed off or spread around if one person is over capacity, but we always make sure the liaison is involved for subject-matter expertise. Or, if the person with the most expertise doesn't have capacity, they at least review the search strategy.
I think this is a problem almost everyone comes up against -- it comes up frequently in workshop discussions. I'd be interested to hear how others handle it too.
Megan