Or search: incoming, invasion, admission, incursion, intrusion, enrolment, irruption, admittance, enrollment, registration
2 Syllables: SU
deadening, deafening, leveling, meddling, reckoning, settling, threatening, trembling, wrestling
3 Syllables: SUU
beckoning, bettering, centering, editing, Evelyn, festering, gentlemen, lengthening, lessening, lettering, levelling, measuring, menacing, mentioning, reddening, rendering, revelling, severing, strengthening, tempering, tendering, threatenings, trespassing, venturing, weathering
3 Syllables: USU
assembling, endeavoring, remembering, resembling, unsettling
4 Syllables: USUU
developing, discrediting, embarrassing, endeavouring, enveloping, inheriting, surrendering

Found 46 words across 4 groupings. Skipped 1 excess word of a similar stem.