To attach to a running job (job $JOBID)
srun --pty --jobid $JOBID /bin/bash
This will run inside the cgroup (CPU, RAM etc.) of the running job.
If you want to see the processes you can run top directly via
srun --pty --jobid $JOBID top
For jobs with multiple nodes you need first find the node you want to attach to via
scontrol show job $JOBID |grep NodeList
Then use the -w switch to specify the node ($NODE)
srun --pty --jobid $JOBID -w $NODE /bin/bash