If we want to use more than 5 users, then we need to upgrade the server to RTM and add users to TFS groups(administrator/contributor, depending on the level of authorization we want to give to the users). For detailed steps in doing so, refer the steps provided in the following article:
How to: Upgrade from Team Foundation Server Workgroup Edition
P.S. In order to determine the number of licensed users you already have in TFS, please follow the steps provided below:
1. In Team Explorer, select the Team Foundation Server for which you want to display group membership information.2. From the Team menu, point to Team Foundation Server Settings, and then click Group Membership.
3. In the Global Groups dialog box, select the Team Foundation Licensed Users, and then click Properties.
4. In User or Group, review the list of member users .