Query? with regards allocating an account


So our accounts all had one user as their account rep, but now we are changing and an account could have multiple reps.

We had a group with all our reps in, and they could only see accounts that were allocated to them by us manually selecting the accounts and ticking allocated to x rep.

Now 3 or 4 reps could all need access to one account, Help, how do i organise that and is there someway to automate it so that i can say if it has x in it. It is allocated to rep A, but if it has x,y,z in the field it needs allocating to rep a, b, and c ?