Adding my vote to the belief that Marten was the only one invited, and his invite was for Marten +1, with the +1 assumed to be Dora. Also, I'm betting Marten got the invite back when he and Dora were still together, so even if Henry did get the news of their breakup (doesn't seem like it in the present comic, Marten's relationship status isn't discussed) it's probably too late to change the invite anyway.
I don't think Henry knows Marten's friends well enough to invite them separately, so it'll be up to Marten to decide who his +1 is going to be. I'll be surprised if it's Claire, quite frankly, Marten's just getting to know her and I don't see any real romantic interest there, at least on Marten's part. And since he and his father were talking about "plane tickets" that means a possibly long trip together and maybe sharing a hotel room or other guest quarters, not sure Claire would be up for that with a guy she's just getting to know. Other choices are between Faye and Hannelore, not sure Dora would want to be away from CoD (and Tai) for a couple days even if invited (yeah I know she was really excited about being a flower girl, but remember she's a business owner who's very reluctant to leave her business in the hands of anyone else, and closing it for a couple days could cost her customers).