Hi Jon Douglas,
I think any item in ArcGIS Online can only have one owner at the time. But you can always change ownership if needed.
So, if one of your team members needs to work on an item, e.g. a hosted feature layer, owned by you, they can take ownership, complete their tasks and - optionally - reassign ownership to you.
I have been working in a team (with multiple administrators) where we followed this workflow of changing ownership all the time.
HTH,
Egge-Jan