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.
There is an Ohr haChaim by the mitzvah of hashavas aveidah which learns homiletically (al derech derush) that returning lost objects to one's friend is an allegory for returning lost Jews to Hashem.
I am not sure how authoritative it is really as a halachic source though.