When should we use embedded documents in MongoDB?

Question

I read a lot about embedding in MongoDB, but still I don't know when to use it. I thought of some scenarios:

I have A collection named UserGroups with these fields:

• _id
• name

Now I want to add an user to Users collection:

1. I should have a groupName field in Users collection and update it when UserGroups collection gets an update.

2. I should add the groupId field in Users collection and perform a join whenever I want the UserGroups.name.

3. I should add the user document as an embedded document to UserGroups collection.

4. Or I should add the userGroup document as an embedded document in Users collection. though, I don't think I should do it.

Which option should I use?