Well, it might be difficult to keep up with the group if you don’t use Genki because the order and exercises will be different. It depends on what the group is going to do together, just talk about learning Japanese and support each other or work on the book chapter by chapter.
But there are plenty of free good grammar resources online at Genki level. Check out the list here: The Ultimate Additional Japanese Resources List! and you might see one that meets your needs best. Tae Kim’s is a fine grammar text in my opinion, but it has a different approach to Genki.
And you also might want something for listening/speaking practice to make up for the lack of audio exercises.
Memrise.com has Genki vocab decks and I’m sure they’re floating around for Anki too, although you might as well just do WaniKani + a better vocab list.
When are you going to university? Your school might have it or another Japanese textbook in the library - almost certainly any copies left over will not be for loan, but it might be handy to have it as a reference to keep up with what the group’s doing.
Are there others in your situation in the group? Genki is an expensive book, especially with the workbook and/or answer key. You could sort out what you’re going to do together.