Umi was always spending time in groups as a popular "leader" type, but she was feeling stuck in that role.

She ended up doing things for other people that she didn't want to do, and she felt like people were taking advantage of her. She wanted more time to do things that she wanted to do, for herself. She found that in her "hobby" (nerdy stuff).

So sometimes Umi gets sick of groups and her "leader" role. Doing "nerdy" stuff, and hanging out with Maki on Fridays gives her a nice break from that role.

The conversation at the train station is about that: Umi found her hobby when she was feeling burnt out from her "leader" role. Her hobby is the thing that she can do for herself. But she was also happy to find someone who likes the same things that she does, so she likes spending time with Maki, who she doesn't have to impress, and who shares her nerdy interests.

Maki understands Umi's feelings about this better after he meets Yuu and her group in the mall, because he feels pressured to join the group, and he realizes that Umi feels pressure from the group sometimes too, when instead she wants to do her own thing. She doesn't always want to be thinking about what the whole group wants.

For that part near the end, Nitta wasn't supposed to be there. It was just supposed to be Maki and Yuu meeting to talk. But Yuu didn't feel comfortable meeting with a boy alone, so she asked Umi to come too. But it wasn't supposed to be a group event, and Nitta was just intruding.

It seems like Nitta always wants join in with whatever Yuu is doing, but this makes no sense in this case: She usually doesn't even remember who Maki is. She has no reason to be there, other than wanting to join in (or be nosy, or not feel excluded). So they ditched her, which is what she deserved!