First: Login to Gliffy. Click Account, Manage Account/Users. In the space next to 'add new users to this account' enter email addresses of those you would like to invite to your Gliffy account. When complete, click the 'add user' button.
Then: Each user you invited will receive an email from Gliffy, inviting them to the multi-user account.
If they already have a Gliffy account, when they login they will be brought to the "Welcome Back" dialogue. They should go to Account Settings->Switch Accounts. They will then be brought to the group account vs. their pre-existing account.
If they do not yet have a Gliffy account, their email will include instructions about how to login to Gliffy, including a temporary password. Their login process will bring them right to the group account.
Users will have access to all files in the account. If you would like to limit the files your employees access, please follow the directions here: http://support.gliffy.com/entries/271515