I believe the people picker on sharepoint list and on smartform does not allow mukitple values to be set via rule.

You can do this. we do this and we do not prepend the claim value we just send ie domainuserID. A people field in sharepoint has two fields in the smart object one is the sme of the field and the other is Name(Value)

1) The sharepoint field must allow multiple selections. This is a field setting in Sharepoint.

2) You will set up the picker control on the smart form to user adservice2ad user. You will set identified to Name and display to whatever you want the user to see but normally that will be DisplayName.

3) On the controle properties choose the field without value for Field and the field with (Value) as the display field

4) On the control properties, you will set the control to "allow multiple" and delmiter to ";" semi-colon.

5) On your create item or update item rule you will drag the entire picker control to the sharePoint field

Sharepoint should with its own internal claims process convert this user ID to the claim value it needs. if not you have two other choices

1) Use the Sharepoint Integration People Picker Source (under systemSharepoint Integration) Set up exactly as above but in this case Name is an actual claim value and not domainuser ID. The only reason I would not do this first is that it is sually much slower than the Ad control.

2) If for some reason neither of the above works you would need to send it to a workflow that could do a "for each" and build the strings that way, then update the sharepoint list item but I dont' think this would be necessary. We have numerous forms that use bioth other methiods above and write to sharepoint 2013 people fields.

I hope this helps