Or search: riding, descend, unhorse, discontinuation
2 Syllables: SU
bounded, counted, founded, grounded, hounded, mounted, pounded, rounded, sounded, township
3 Syllables: USU
abounded, accounted, amounted, astounded, compounded, confounded, recounted, resounded, surmounted, surrounded, unbounded, unfounded

Found 22 words across 2 groupings. Skipped 1 excess word of a similar stem.