Here is an example from browser REST client that adds four test cases to a test set:
endpoint:
https://rally1.rallydev.com/slm/webservice/v2.0/testset/14469862335/testcases/add?key=fbaa0c04-...
Payload:
{
"CollectionItems": [
{"_ref": "/testcase/14469885324"},
{"_ref": "/testcase/14469885842"},
{"_ref": "/testcase/14469886070"},
{"_ref": "/testcase/14469887154"}
]
}
See also Collections in WS API.