While you can't invite them back, if they come back and the group finishes the skirmish, they're still eligible for rewards. It happened to me tonight. I DCd about halfway through Locke's Lie and when I came back my party was unable to invite me (I was still in the instance and helped fight). We continued on and finished it and I got a Giantsgall Longsword even though I DCd and wasn't in the party at the end.