How do I get check numbers in the register?

I don't know what you mean by "for a group" - but assume you mean that you set up a new bank account?

To change what columns display in your register, right click any of the register column headings to get a list of available fields ... just check the ones you want.  A bank account should have had the check number field visible by default I think.

Notice the "Apply to all Cash Accounts" (as opposed to Investment) - so you can quickly have the same columns visible for all accounts.

Screenshot is from Quicken Mac version 6.9.0 (current).