How can I get help taking G train from shenzhen North to Wuhan?

Well, it is generally fine to buy the train ticket in Shenzhen North Station or any train ticket office in that city. But if you will travel during Chinese holidays, like National Holidays and Spring Festival, tickets may have been sold out ahead. In this case, you'd better book the ticket in advance.

By mentioning "join a group traveling there", I wonder whether you refer to joining a tour or group to take a train? For the former, you can turn to a travel agency like Travel China Guide. They may arrange that for you. For the latter, I'm afraid it is hard to achieve. Anyway, the signs and broadcasting in the train stations are both in Chinese and English You won't get lost. Relax!