How to set relationship when I create a new (custom) content to another (custom) content in Drupal
I hope I'm understanding your question correctly... you'd like to know what's the best way, in Drupal 7, to add a reference to a node of another content type when creating this content type?  (i.e. to store a reference to a 'Group' when creating a new 'Person')? 

If that's what you're trying to do, a good way to do that in Drupal 7 is using the Entity reference module.  

Once you download and enable this module, you'll be able to go to the 'Manage fields' tab of your 'Person' content type and select to create a new field of type 'Entity Reference'.  That field option will only appear once you've enabled the module.  Before hitting 'save' after you've selected that field type, make sure you notice that you'll have two options for the field widget: you can select for it to be a 'Select list' in which case, it will just have a drop-down list of all the Groups currently available on your system for you to choose from when creating a person.  Or you could choose 'Autocomplete', which works better if you have a TON of groups and don't want a super long list.  That'll allow the user to start typing the name of a group, and it will just automatically detect and complete it.  :-)

Once you've selected those two options on the first screen, on the second screen you should select 'node' as the 'Target type' and select 'Group' as the type of the referenced node.  Finally, just choose whether you want that field to be required or not and whether you should select only one or multiple values (like any other field).

Let me know if that works for you, and I hope I understood your question correctly!!