Editing entity subgrid via query

Hello,

I'm wondering if there is a way to modify a subgrid within an entity form using web api. I need a way to automatically change the contents of the team subgrid within the systemuser entity form from a separate dasbhoard. 

Or I need to know how to automatically create teammembership records (via web api POST or Xrm.WebApi.createRecord) to establish the relationship between systemuser and team. 

Thank you very much,

Scott