Starfleet Regulation 191 (Article 14): In a combat situation involving more than one ship, command falls to the vessel with tactical superiority, should there not be a higher ranking officer present.
I'd say of all the ships in the fleet engaging the Cube, the E-E has tactical superiority, giving Picard more than enough justification to assume command. Add to that he was the previous CO of the Federation Flagship, I'd think most Captain's would line up behind him, regardless of how he came to be involved.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)