3rd person plural present indicative (or nominal form with abstract -ti); the surface form saṁvijjantīti is contracted, with -antī (feminine plural present participle) + ti (quotative/anaphoric particle), 'saying (that) they exist'
Analysis. saṁ + vijjati: 'together + to be found/exist'; the -ti ending here marks a quotative or cited speech clause, 'saying that...'
Etymology. saṁ (together; same root as English 'same') + vijjati (from √vidh, 'to find, discover, exist'; related to Sanskrit vidyate and ultimately to Indo-European *wid-, 'to see', as in English 'wit' and Latin 'videre'). The concrete image is things discovered or appearing-together.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
Lite mode — plain HTML for e-readers and old browsers. Full version