A Private IP Pool can be defined for your Stateful Elastigroup in the group's configuration, under the Compute tab's Stateful section. When you select the "Maintain Private IP" option, you may also check "Assign Specific Private IPs":
When assigning several Private IPs, the group will create an SSI (Stateful Spot Instance) in a Paused state for each Private IP.
You may then launch any of those instances by using the Resume action (for more info on stateful actions click here).
If one of the SSIs in the group is deallocated, a new Paused SSI will be created to replace it automatically, in order to maintain a match with the Private IP Pool.
Removing an IP from the Private IP Pool:
When an IP is removed from the pool (or the option is deselected entirely) any SSIs that were created for each IP and never Resumed, will be Deallocated.