Assistant professor does not collaborate with or cite fellow department members, despite them being experts on the topic I serve on a Promotion and Tenure committee for an assistant professor who has (in my opinion) a lack of professional courtesy and collaborative attitude. This professor will collaborate and publish with graduate students or faculty at other institutions. 

However, has conducted at least 5 small research projects, that have been published in regional or national research conferences, where faculty in our own department who are experts in the field were not included and even their previous published research not referenced (I think there's a clear issue with reviewers, but). 

Only after it is clearly and sometimes loudly pointed out "you did this whole project and didn't even reference any of XXX's work or ask them to be part of the study in his/her area" will this faculty member ask people to review the article for journal submission. 

Further, there are a few faculty across the country, who could be external reviewers for this individual. They have also witnessed this lack of collaboration and professionalism. 

While collaboration is not a measure of success in the P&amp;T process, the quality of the research conducted and published should be. 

My questions are:

How do I guide, mentor this young faculty member through the promotion and tenure process, so that they understand the importance of collaborating with the experts within the department? 

How do I address this issue during the P&amp;T meeting with the rest of the committee?
You seem to think that an assistant professor should collaborate within your department.  However, a more common view is that an assistant professor should be an independent researcher.  An assistant professor should clearly distinguish their research from that of other faculty at the same institution.  Assistant professors do not want to be asked, "Why should we grant you tenure when we already have a tenured professor who can do the same research?"  

Without more detail added to the question, it sounds like the assistant professor's collaboration strategy is okay.