Is kiruv a mitzvah? Is it biblical or rabbinic? Is there any reason it should supersede other mitzvot? what are the sources in chazal and halacha relating to kiruv?
related: Kiruv and halachik compromise
Rav Sheinberg ZS"L said the source is "Kol Yisrael Arevim ZeLaze."
The mitzvah of tochecha(rebuke) and vahavta l'reacha kamocha(loving ones friend) are intertwined and kiruv is a huge mitzvah and it is like saving a life. Rambam (Mitzvos Aseh 3),Minchas Chinuch (Mitzvah 239 Os 4) .The Gemara in Shevous 39 brings the idea from a passuk of areivus(responsible for a community)that not only one has to help individuals but also bring a community closer.
Kavod Shamayim - Honor of Heaven:
Arvus - All Jews are guarantors for each other:
Hashavas Aveidah - Returning a lost object:
Hakamas HaTorah - Upholding the fulfillment of the Torah:
The sources I didn't link to are readable in the above source sheet.