Hazelcast Jet supports the scenario where a new member joins the cluster while a job is running. Currently the ongoing job will not be re-planned to start using the member, though; this is on the roadmap for a future version. The new member can also leave the cluster while the job is running and this won't affect its progress.
One caveat is the special kind of member allowed by the Hazelcast IMDG: a lite member. These members don't get any partitions assigned to them and will malfunction when attempting to run a DAG with partitioned edges. Lite members should not be allowed to join a Jet cluster.