T'Challa was there, of course! That Wakandan wine is legit. It's pretty strong too, even for a super soldier-type so tread carefully~
He was very excited to meet Yelena, and I presume he did know about her (and the rest of Natasha's family) since they were dating for awhile before he disappeared, and probably talked about families - though Zip can correct me if I'm wrong! But either way, he'll chat with her and also with Sam and whomever else was there, then go meet with Natasha later someplace else in the Tower.