In RFID systems, tag grouping is to inform each tag in the interrogator's RF field which group it belongs to by assigning them a group ID. With this information, the reader can write the same data into a group of tags in one transmission by taking their shared group ID as the destination address, which plays a vital role in improving inventory efficiency. Existing work proposes some efficient protocols but suffers from two defects: slot waste and data redundancy. To address these problems, we in this paper propose a two-phase grouping scheme that improves the grouping efficiency via slot reuse as well as data compression. For slot waste, we try to reconcile collision slots and turn some of them into useful again by re-assigning a new slot to collision tags, which increases the number of useful slots and reduces slot waste. For data redundancy, we encode group IDs with lossless coding algorithms and reduce the data redundancy caused by repetitive transmission of group IDs. Two coding methods, Huffman coding and arithmetic coding, are adopted to shorten the length of the bit vector comprised by group IDs. Theoretical analyses and extensive simulations show that our best protocol improves the time efficiency by 51.9 percent compared with the state-of-the-art.