Query? with regards allocating an account

Hi.

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 ?

Thanks