How would you say "In this sign love" as in the similar style saying of "In hoc signo vinces"?
The phrase in hōc signō vincēs, as in the words Constantine saw, uses a future indicative form: it's not an order to conquer, it's a statement of pure fact. "In this sign you will conquer." The word you'd want to change is vincēs.
If you want to keep that same grammatical form, "you will love" as a statement of fact, that would be amābis in the singular (like vincēs), but amābitis in the plural.
If you want it to be a command ("love!"), that would be amā singular, amāte plural (the present imperative). And if you want it to be a suggestion or a wish, "may you love", that would be amēs or amētis, the present/future subjunctive.