add members to a file/folder

You don't need to use DbxRawClientV2. You should instead use DbxClientV2, as shown in the examples, e.g., here.

Calling addFileMember would look like this:

DbxRequestConfig config = new DbxRequestConfig("example");
DbxClientV2 client = new DbxClientV2(config, ACCESS_TOKEN);

List<MemberSelector> newMembers = new ArrayList<MemberSelector>();
MemberSelector newMember = MemberSelector.email("dbapitesting@gmail.com");
newMembers.add(newMember);

List<FileMemberActionResult> fileMemberActionResults = client.sharing().addFileMember("/test.txt", newMembers);
System.out.print(fileMemberActionResults);