Donna will absolutely be there. She loves weddings. Sends in her RSVP as soon as she got the invite. She can't freaking wait.
Marcus will go. He remembers Wanda from that day everybody got the horrible headache. He's happy to go and see her and Vision tie the knot. He sends his RSVP back in a timely manner.
Dean checks it but forgets about it until the day before it's due. But he RSVPs too. He's never been to a real wedding. Might as well go and check it out.
Olivia isn't sure at first. She doesn't really know them but then reasons if she was invited, then it would be sort rude not to go. Also, it would nice to meet new people in a cheerful environment. She RSVPs after a few days. She'll go as long as she's not out in space at the time, and she'll let someone know if that's the case.