UKMitel is correct that ring groups are not supported at your software revision.
The multilinbe key would be your best method.
From the release notes for 8.0
Ring Groups
Ring groups are often used in small businesses to provide an ultimate answer point for callers, rather than directing incoming calls to specific individuals and then using call forwarding or call re-routing. In Release 8.0, Mitel introduces ring groups which can operate in one of two modes:
Simultaneous Ring – where all extensions designated in a group ring at once.
Cascade Ring – where the call is presented to the first member of the group, and if they fail to answer the call, it then cascades through the other members in the group.
Within our implementation, calls can be queued at the ring group and will be answered on a longest ringing-waiting algorithm. Where calls are queuing for an excessive period (based on a programmable timer) or where the ring group is unavailable, an overflow point can be programmed to ensure calls are serviced as efficiently as possible. For some small businesses ring groups may provide a simpler alternative to ACD.
For flexibility, devices can exist in multiple ring groups. In this case, if calls are queuing at both groups a device is a member of, then the longest waiting call will be presented to the device when it becomes free (so unlike ACD, there is no concept of higher priority groups / queues). Note also that ring groups are different to ACD groups in that users are not required to log in and out of the group.
Members of a ring group can include display sets, non-display sets, single line phones, multiline set prime or non-prime numbers, night bells, and hot desk user IDs. Current exclusions include remote directory numbers (i.e., ring group members have to be programmed on the same 3300 ICP as the ring group), operator consoles, speed dials or ARS strings, and ring group pilot numbers (so ring groups cannot be nested).
Specifications include:
A device can be programmed in up to eight ring groups
A ring group can have up to 32 members
A maximum of 176 ring groups can be created
*******************************************************
Occam's Razor - All things being equal, the simplest solution is the right one.