If performance slows noticeably while many users are logging on, you probably need to add backup domain controllers (BDCs) to help process logon requests. Any Windows NT Server can be used as a BDC.
In some instances, for example if browsing the network is slow, you might also want to divide the network into more domains. For a discussion of strategies to use in organizing your network into domains, see "Knowing the Domain Models" in Chapter 3, "How Network Security Works" of the Windows NT Server Concepts and Planning Guide.