Have a look at the Node access user reference modules (with about 4,3K reported installs ...), available for both D6 and D7.

Some details from its project page:

Gives content access permissions to users for content that references the users with User reference or Entity reference.

 

This is great if you want your content authors to be able to choose additional authors, or choose who can view their content, as well as being able to display the list of authors or viewers within your content, or in views displays.

The above details seem to me like a perfect solution to answer your question. So either just use this contributed module in combination with your own module, or use it a inspiration about how you should enhance your own module in combination with User reference or Entity reference.

Have a look at the Node access user reference modules (with about 4,3K reported installs ...), available for both D6 and D7.

Some details from its project page:

Gives content access permissions to users for content that references the users with User reference or Entity reference.

This is great if you want your content authors to be able to choose additional authors, or choose who can view their content, as well as being able to display the list of authors or viewers within your content, or in views displays.

The above details seem to me like a perfect solution to answer your question. So either just use this contributed module in combination with your own module, or use it a inspiration about how you should enhance your own module in combination with User reference or Entity reference.