Creating content types with fields that are other content types?
In future it would help to know what version of Drupal you are using.

In D6 you can use Node Reference CCK field, which creates a cck select list, auto complete, radio buttons or checkboxes listing the node types you have selected to be referenced.

There you could use your autocomplete form, type the users name and add them to the project.

This module however no longer exists in D7, instead have a look at these modules which could do the same or similar: 

http://drupal.org/project/references

http://drupal.org/project/Relation

http://drupal.org/project/entityreference