I would like to have a workflow fire each week to update a group that the on-call person has changed and what the changes are
Please search the community for Site workflows and Scheduling Site workflows? It may help.
As Kapil Joshi says, this would be possible with scheduled workflows. Please refer to this community post: Schedule a Nintex Workflow for SharePoint . The help files go into this in additional detail - http://nintexdownload.com/helpfiles/nw2013/NintexWorkflow2013UserManual.html#Nintex.Workflow.SchedulingWorkflow
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