Usually our Lead Investigator gives us what information she knows and we go with it. I would guess how much info you give out to 'whom' would depend on the members of your group. There are, in my observation, two basic types of investigator. Sensitives and Technicals.

SENSITIVES
Tend to fall into the psychics, and people who hear, see or feel things and people who want to believe all hauntings are real. I am not saying these people are uncredible. I have met some people that can do some pretty fascinating things from moving objects without touching them to telling you what you will be doing in a week. I have met people who really saw things that were there. However in this group are also clumped the people who seem to see everything everywhere and you are never quite sure if they are really telling the truth or want attention. Everything can't be everywhere.

Investigators that are psychic or have specialized acute tuning to see, hear, or feel things, shouldn't be told much more than a basic outline of information. I would refrain from specifics. Let them know there was a death or trauma in the house but withhold sex, names, specific rooms - details like that.
Knowing too many details going in could easily hamper honest impressions and the credibility of the investigation.
See 'whats-his-name' from Most Haunted for a good example of a 'psychic' knowing the history before the investigation.

Another type to probably not give the full story to are those people who tend to easily believe every house is haunted, people who always say they 'see' things, or 'feel' things when the rest of the groups general consensus is the property owner is simply afraid of their shadow. Giving them all of the details of deaths in specific rooms can feed the fire and whip them into a froth.
A few friends and I jokingly refer to these people as 'Brians'. It is our secret code, which refers to Brian from TAPS who tended to be a little too wide eyed and willing to believe every blip is a haunting. These people also need to often be reined in as they tend to get excited and talk about personal experiences in front of the property owner which is a no no.

TECHNICALS
These people tend to be more on the reserved side and try to figure out the cause of things before going "GHOST!" and then looking like idiots later because of a tree branch.

The investigators in this group that tend to be more skeptical, analytical and technical, should have the full story and details - and should be told specifically to not to share these details to the two groups mentioned above. These people will be on the look out for causes - tree branches, freeways, lights, noise from the neighbors, etc. They will know where to set up the equipment and what to watch for. These are also the people you want to mix with your 'Sensitives'.

When the post investigation logs are turned in (if you do them) it will be easier to maintain a balance and get a better impression of the overall investigation. After the investigation you can then meet, share all of the details and have an open floor discussion. At this point you have everyone’s logs turned in and accounted for and the integrity of the investigation will still remain intact.

Integrity and credibility is a very fragile thing in our field. The skepticism I get when I tell people what I do for fun is very strong. So we travel a fine line between Medicine Show Salesmen and Knights in Shining Armor. Fielding the information you under share or over share to certain individuals in the group will help maintain the reliability of the information you end up submitting to the hosts during the post investigation.

Hope that helped :)