While collaboration is not a measure of success in the P&T process, the quality of the research conducted and published should be.
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?
You seem to be contradicting yourself here. If collaboration is “not a measure of success in the P&T process”, then you can’t expect the faculty member to “understand the importance of collaborating with the experts within the department”. Either collaboration is an official criterion or not: if it is, explain this to the tenure candidate. If it isn’t, you have no business viewing it as a weakness of their tenure case, and neither do the other faculty members you mention who are “mad about the lack of inclusion”. Quite simply, there is nothing to “understand”.
How do I address this issue during the P&T meeting with the rest of the committee?
You say you are concerned about the tenure candidate’s “lack of collaboration and professionalism”. Perhaps a way to address it is to give some thought to your own professionalism in advance of the meeting. Part of professionalism is following the policies and procedures of your institution without allowing your judgment to be clouded by irrelevant factors. Since you said that collaboration is not an official criterion, I suggest that you focus only on the factors that are official criteria, and be prepared to remind your committee colleagues of what those factors are and steer the discussion back to them in case any of them attempt to bring up other, irrelevant factors.
Another aspect of professionalism is not tolerating your department functioning as a kind of mafia in which senior faculty members exploit more junior ones by pressuring them into entering collaborations and into citing the senior members’ works, and trying to make the junior members “understand” that such collaborations are “for their own good”. I understand that I may be reading a bit too much into your description and maybe that’s not what’s really happening here, but it does sound like such a coercive/exploitative environment is a possible interpretation of what you wrote.
Since you say that the quality of the candidate’s research is important in the P&T evaluation, what would be a good idea is for you to counsel the junior faculty member about ways in which they could improve the quality (and quantity) of their published research. If you truly believe in good faith that collaborating with senior faculty members in your department would be a good way to achieve that goal, it would certainly be reasonable of you to explain that to your mentee. It would also be reasonable to discuss with them academic standards involving citation of relevant literature. If there is a real issue of them not including citations to clearly relevant earlier work in their publications, definitely this can be an important issue that you can and should discuss with them. However, it should not matter whether the literature they are not citing is by someone from your department or outside of it. The only relevant issue is whether your colleague is living up to the high standards of conduct and scholarship expected from a tenured professor at your university.