Unmodifiables groupingToAll
Instead of groupingBy that groups each item on a single key, can group each item to a set of keys. Otherwise as grouping by, but each item can come under multiple keys in the result.
List<Issue> source = [...];
Map<Integer, List<Issue> byUser = groupingToAll(i -> setOf(listOf(i.getIssuer(), i.getAssignee(), i.getVerifier())));