The phrase "assemble before" is correct and usable in written English.
It can be used to indicate that a group of people should gather or come together prior to a specific event or action.
Example: "Please assemble before the meeting starts so we can discuss the agenda."
Alternatives: "gather prior to" or "meet before".
The people of each cell are supposed to assemble before these judges on a patch of grass (gacaca).
The bill is one of the final pieces of the legislative puzzle Congress must assemble before a holiday recess.
Instead, there is a large meeting hall in which families and other gatherings assemble before being taken into the galleries or classrooms.
Apart from the financial problem — already "solved" — the chief difficulty was the great number of papers every emigrant had to assemble before he could leave the country.
Shortly before half-time Wakaso Mubarak doubled the lead, whereupon the area around the tunnel was peppered again and Ghana's players were made to wait for police to assemble before departing the pitch.
There are 2,000 caddies on duty: a red-uniformed army, battalions of which are always standing at parade rest in a staging area where golfers assemble before deploying to the courses.
McCain credited the coalition of supporters, "not ever assembled before," for helping push the Senate to act.
Nothing quite like it has ever been assembled before, the foot bones connected to the ankle bones and everything else up to the cranium.
This environment implicitly imposes ordering constraints on the self-assembly process, where certain parts of the target structure must be assembled before others, and where it may be necessary to assemble (and subsequently disassemble) temporary structures such as staircases.
The scene began to have the look of troops assembling before battle.
He then addressed those assembled before him: "My fellow Americans", he intoned, "God bless America".
