To add a table to a rich text document place your insertion caret, then choose Format > Table. An empty table is inserted and the standard OS X RTF table panel appears that lets you change e.g. the numbers of rows and columns.
Place the caret in any cell to change also the alignment, its border, and its background. Use Tab and Shift-Tab to move from cell to cell. If you have closed the Table panel and want to bring it back, place the caret in any cell, then choose Format > Table again.