We investigate strategies for user cooperation in the uplink of a direct-sequence code-division multiple-access (DS/CDMA) network. We consider two repetition-based relay schemes, namely decode-and-forward (DAF) and amplify-and-forward (AAF) and we provide performance analysis of various exact and blind linear multiuser detection strategies in terms of the (asymptotic) signal-to-interference-plus-noise ratio (SINR) and the bit error rate (BER). Our results show that, on one hand, AAF is more robust to the multiple access interference (MAI), achieving better performance than DAF, especially if a simple matched-filter (MF) receiver is employed at the relay side. On the other hand, DAF is more amenable to blind multiuser detection when some system parameters are unknown at the base station (BS).