What I have done in the past, when managing SfO for about 350 users, was to be religious with my own personal spreadsheet which logged every SfO version a user was on and their PC name at the time. But you probably can get most of the users. The blind spot here though, is that you assume that all users who have SfO, would be using it. For example '34.0' or '281.0' refer to 3.4 and 2.8.1 SfO versions respectively. Any user which has a ClientVersion value populated against it has that version of Salesforce for Outlook. You will need to do a vlookup against the UserId to return the usernames but you will then see a grouped list of users by their login usage (count)id and their ClientVersion. This is because the sObject LoginHistory is very special (aka 'annoying') and there are a number of limitations on this object. Unfortunately, one cannot filter on 'Application', so you can't say 'WHERE Application = 'Salesforce for Outlook'. So what else can you do? This link is quite helpful and the least tedious way of achieving this would be to login to workbench or the Query Editor in the Developer Console and run this SOQL: *SELECT count(id), userid ui, ClientVersion cv FROM LoginHistory GROUP BY ClientVersion, UserId* So, if this were the case, you would be back to square one. Obviously, people move around desks a lot etc.
The blind spot here, in my experience, is that although the IT team will be able to get a list of all the PC names which have a SfO app installed and the corresponding version(s) (a machine may have more than 1 different version of SfO installed on it), they may not know to which that PC name belongs to. msi version is generally deployed by the internal IT team and in order to achieve this, the IT team need to know the PC names (on Windows, it is My Computer | System Properities | 'Computer Name') for the users who will get this app installed. Second best option is to query the LoginHistory sObject via Query Editor or Workbench. And if they don't have a fully up to date list of which PC name belongs to which colleague, then you're essentially back to square one. This is an interesting question and the short, and painful, answer is that there isn't a fully accurate way of getting this data without assistance from your IT department, who should have/should be able to create a list of PC names which have Salesforce for Outlook apps installed on them and the corresponding versions.