Entrar escalonadamente en un edificio

Entrar escalonadamente en un edificio
 

How can we say escalonadamente in this context? thanks

If what you write means means, entrar el edificio por las escaleras you would say, enter a building using the stairs.

enter a building step by step ??? or just enter a building gradually ?

you can say: to slowly enter a building step by step. Entrar un edificio lentamente paso a paso.

I mean, a group at 10, another group of 5 at 10.20 etc....

Enter a group (of 5) at a time, twenty minutes apart.
You can change up the numbers as needed.

Or, without specifics beforehand:
Enter by groups, as directed.