Can you play vampires from Camarilla and Sabbat sects together?

Hey we are in a game and one of our players has a crypt mixed with both camarilla and sabbat sect vampires. We thought we remembered some rule saying you could only play the same sects together?

There is no rule that prevents you from mixing vampires of different sects.

I guess you're confusing it with the grouping rule. Vampires have a group number (on all but very old cards it is printed just above the text box). They don't interact with the game itself, but when choosing vampires for your crypt you may only choose from 2 adjacent groups (e.g. 3 and 4 is all right, 4 and 5 is all right, 3 and 5 is not).

please bear in mind that antitribu and regular clans are mechanically distinct clans in VTES. So you can't use a Ventrue to play a card that requires Ventrue antitribu.