So you are using NO version control? With that many users and objects I would have assumed that you would have experienced at least some significant problems by now?
We have experienced no problems to date and are comfortable with this approach so far because of 2 reasons:
1) Users are clear on which package they are working on and there is never a case when 2 people are working on the same element at the same time. We manage this by deciding on package structure up front and having a designated sub-team member responsible for each package. for e.g. Lead BA for Use Case model, Technical lead for Component model etc. Lead BA is requested when new sub-packages are required and BAs are assigned to work on groups of work within a package which is based on logically related stuff.
2) We keep frequent snapshots of our model as XMI Exports named by date, so we can always compare with current model status and highlight the differences.
Our SQL database, on which this model sits is backed up daily. So that gives us an added level of security, that should the DB go down, we are able to get it restored to a suitable state.
Like I mentioned, we initially turned on the version control but soon found it annoying and requiring constant management and hence decided to adopt the current approach.
My suggestion, try both and see which works for you. But if you are nervous start with the version control and get rid of it once you are familiar with the product and decide on the ideal working style for your team.
HTH.