Ah, ok. You need to be a JIRA or system admin, so log in with one of those type accounts, then go to Admin -> Custom fields, and find the "reviewer" field in the list.
To the right, look for "configure" for the field and open that up. You should find at least one "context" for the field, which I suspect will include something like "only show users in group X"
I'm sorry, I'm not sure where you are looking. Could you follow this and tell me where you get stuck:
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