Wireless power transfer and backscatter communications have emerged as promising solutions for energizing and communicating with power limited devices. Despite some progress in wirelessly powered backscatter communications, the focus has been concentrated on backscatter and energy harvester. Recently, significant progress has been made on the design of transmit multisine waveform, adaptive to the Channel State Information at the transmitter (CSIT), in point-to-point backscatter system. In this paper, we leverage the work and study the design of transmit multisine waveform in a multiuser backscatter system, made of one transmitter, one reader and multiple tags active simultaneously. We derive an efficient algorithm to optimize the transmit waveform so as to identify the tradeoff between the amount of energy harvested at the tags and the reliability of the communication, measured in terms of Signal-to-Interference-plus-Noise Ratio (SINR) at the reader. Performance with the optimized waveform based on linear and nonlinear energy harvester (EH) models are studied. Numerical results demonstrate the benefits of accounting for the energy harvester nonlinearity, multiuser diversity, frequency diversity and multisine waveform adaptive to the CSIT to enlarge the SINR-energy region.