Assigning Existing Group IDs to Group Members

[robboll]

Problem: Assigning a pre determined ID to members of three groups.

Given --Three Groups:

Group1 (members 1 thru 7)
Group2 (members 1 thru 5)
Group3 (members 1 thru 4)

Their corresponding (but unassigned) IDs are:

G1_1
G1_2
G1_3
G1_4
G1_5
G1_6
G1_7

G2_1
G2_2
G2_3
G2_4
G2_5

G3_1
G3_2
G3_3
G3_4

Whenever each member of a group gets selected he gets assigned one of
these group IDs. Say 3 get selected from Group1. they should be
given the first three Group1 IDs: G1_1, G1_2, and G1_3

Then, as a random occurence, 2 members from Group3 get selected, and
are assigned G3_1 and G3_2.

My question is: What mechanism can be used to assign the IDs so that
they are systematically and sequentially assigned?

If the last 2 get selected from Group1, they would get G1_4, and
G1_5. I hope this is clear enough.

Thanks for any help with this.

RBollinger