What I would recommend is falling back on policies and procedures. At my current job, there is a policy surrounding PTO(Paid Time Off). We have to submit a request a minimum 2 weeks prior to the date we are requesting off. This gives everyone notice of the absence and allows for proper planning in the individual’s absence. If there is not a policy regarding this, I would suggest making one. The policy should also include a statement about critical points in the project, and the number of people in one group able to leave at one time.