How to find out the current highest index of an array in the flist?

 nap(13382)> robj - 0.0.0.1 /group/sharing/charges 786539
# number of field entries allocated 20, used 19
0 PIN_FLD_POID           POID [0] 0.0.0.1 /group/sharing/charges 786539 16
0 PIN_FLD_MEMBERS       ARRAY [8] allocated 20, used 2
1     PIN_FLD_ACCOUNT_OBJ    POID [0] 0.0.0.1 /account 785579 9
1     PIN_FLD_SERVICE_OBJ    POID [0] 0.0.0.0 / 0 0
0 PIN_FLD_MEMBERS       ARRAY [9] allocated 20, used 2
1     PIN_FLD_ACCOUNT_OBJ    POID [0] 0.0.0.1 /account 785579 0
1     PIN_FLD_SERVICE_OBJ    POID [0] 0.0.0.1 /service/ip 783275 8
0 PIN_FLD_MEMBERS       ARRAY [10] allocated 20, used 2
1     PIN_FLD_ACCOUNT_OBJ    POID [0] 0.0.0.1 /account 732996 11
1     PIN_FLD_SERVICE_OBJ    POID [0] 0.0.0.0 / 0 0

For example:  I have sharing group above and I want to add another member array, in opcode programming how to find out the current array index, that  is 10?  so that I can add one to the current highest index and add an array element to the flist.