The DNC paid Fusion GPS to create the dossier — proven. The dossier was used to acquire a FISA warrant for Page — this will be proven when the underlying documents of the memo are declassified. Under the rules of FISA warrants, anyone connected to the target, Page, is also a target, so that includes Trump and his campaign team.
If it can be proven that the DNC and the FBI were involved in using the dossier to obtain the FISA warrant, knowing it was fake, for the purposes of spying on Trump, that could be considered collusion by the …