Try using the UserManager with the Python API. The UserManager has an option for max_records which can be set to the number of users in your group
from arcgis.gis import GIS
import arcgis
gis = GIS("https://www.arcgis.com", "USERNAME", "PASSWORD") # set first arguement to your Portal or AGOL
users = arcgis.gis.UserManager(gis)
# get the total number of users in your AGOL account provided you have administrative priveledges
totalUsers = users.counts('user_type', as_df=False)[0]['count']
# use the total number of users in your AGOL Account as an arguement for max_users
allUsers = users.search(query=None, max_users=totalUsers)