I can find the answer using brute force as 12, but what is the formula to calculate this for any combination of person and chairs.
Here is the brute force combinations for 2 person, 4 chair:
Group where A is always placed before B
A,-,-,B, A,B,-,- -,-,A,B -,A,-,B A,-,B,- -,A,B,-
Group where B is always placed before A
B,-,-,A B,A,-,- -,-,B,A -,B,-,A B,-,A,- -,B,A,-