The use of opportunistic multicast scheduling (OMS) for systems with multiple multicast groups is examined in this work. Here, a general downlink scenario where a single basestation is to transmit independent data streams to multiple groups of users is considered. Within each group, common information is transmitted, but between groups, the source information is independent. In the literature, the OMS scheme has been proposed for the case where all users belong to a single multicast group. This scheme allows the base-station to transmit at a higher rate in each time slot by scheduling the transmission to only a subset of users in the group. By encoding the data stream using fountain codes, each user will be able to decode the message whenever a sufficient number of data bits are received, regardless of which specific time slots the user was able to receive in. In this work, the use of OMS is extended to systems with multiple multicast groups and the so-called multicast throughput region is defined to characterize the performance of the multigroup OMS scheme. The analytical results based on extreme value theory are utilized to accurately predict the optimal multicast group-sizes and the optimal power allocation policy when maximizing the weighted sum throughput. By choosing the weights appropriately, it is shown that the method can be further utilized to ensure proportional fairness among the multiple multicast groups. The efficacy of the proposed OMS schemes is shown through numerical simulations.