There's a lot of models to draw from. I have personally worked as part of a team of NP's in a critical care service in three different settings.
The first group I worked with was within a Surgical Critical Care Service covering a 16-bed Cardiothoracic Surgery ICU (CTSICU). There were 8 of us providing 24/7 staffing to the unit under the direction of a dedicated group of Trauma/Intensivist Surgeons who take week-long calls to round in the ICU. There are residents from the General Surgery residency program who rotate in this ICU and work alongside the NP's. There is a Trauma fellowship program but the fellows do not typically rotate with the CTSICU service. The NP's follow a rotating days and nights schedule of 3 twelve hour shifts a week. NP's (and attendings) are employed by the hospital's closed medical group model. NP's are paid hourly with provision for shift and weekend differential, holiday and overtime pay. Full hospital-sponsored management level benefits are offered. The nature of the patient population required the NP's to be credentialed by medical affairs to insert all types of central venous lines (multi-lumens, introducers, Quinton's, Swan-Ganz) and arterial lines, place chest tubes and perform thoracenteses, and perform bronchoscopies.
The second group I worked with was a 20+ bed combined Med-Surg ICU in a community hospital setting. There are no residents. The NP's work with Pulmonary and Critical Care trained attendings. It was a new group that was just beginning to recruit a full team and there were 3 NP's at the time I started. We only worked 3 12-hour day shifts a week. The nighshift was covered by moonlighting physicians who are in training for Pulmonary-Critical Care. I took this job as a temporary assignment at the request of the medical director as I had previous experience as an NP in ICU. The director knew I was going to leave in six months to move out of state. We were credentialed for the same procedures as the previous setting I worked in but being in a community hospital setting, the procedures were more commonly central lines, arterial lines, and intubations.
My current job is with a large group of 12 ACNP's within an academic medical center's Division of Critical Care Medicine. The division is very much interdisciplinary. Our attendings have faculty appointments and have a variety of backgrounds: Anesthesiology-Critical Care, Pulmonary-Critical Care, Nephrology-Critical Care, Trauma Surgery-Critical Care, Neuro-Critical Care. Residents come from diverse programs: Internal Medicine, Anesthesiology, Emergency Medicine. There is also a fellowship program. The NP's cover 2 separate 16-bed Neuro ICU's, 1 combined 16-bed Med-Surg ICU, and 1 16-bed combined Cardiovascular Medicine, Vascular Surgery, and Cardiothoracic Surgery ICU. We provide 24/7 coverage with NP's rotating between 3 12-hour nights and day shifts a week. NP's are members of the hospital's collective bargaining group, are benefitted as hospital employees and are thus paid hourly. Some of the NP's hold Clinical Associate Professor appointments in the nursng school. We are credentialed to place invasive lines, intubate, and provide conscious sedation.
I guess the common thread for me was determining how many NP's are really needed to fully realize a 24/7 staffing model with residents added to the equation. I don't know if you will have residents on your team or it will be all ACNP's. The other piece is credentialing - you'll have to determine what you are willing to have the ACNP's do in terms of procedures and how to go about training the team to keep everyone's skills up to par. Fresh graduates of ACNP programs have some exposure to typical ICU procedures but are definitely not proficient. Another piece to consider is billing. I never really have to worry about benefits as I always worked for the hospital and having worked at the bedside previously as an RN, we are quite familiar with how hospital sponsored benefits go.
If you search online on PubMed, there's a good number of ICU NP groups across the country. Some of my current co-workers have come from ICU NP groups from other states like myself. Two of our senior NP's published our experience on ICU Director (a journal for the management side of ICU's). I will not post a link for the sake of privacy but I could certainly help point you to resources should you need one.