Difference between user groups and users

Difference between user groups and users

I am a new user to Zoho.  I have created  project and note that there is a message on the project screen that the project is not associated with any groups.  I have users set up who can be assigned tasks.  What is the advantage of using a group?  How can a group be constructed from current users?  Also, I see that groups can be created from My Contacts but I do not see a way of populating contacts.  Thank you for your help.