formula to generate a string

Hi,

I am trying to build a relationship table from a flat table and should look something like this. The number in column A is considered a parent product and the relations in the result column should be the other products which have the same group values.

It should look like this:

A	B	C
Number	Group	result
1223	110001	1224
1224	110001
1225	100502	1226 | 1227
1226	100502
1227	100502
1228	110720	1229 | 1230 | 1231
1229	110720
1230	110720
1231	110720

The column result obviously needs to be populated with a formula, but I cannot seem to figure out what the formula should be.
I have tried a countif(B:B,B1) formula and copy that down. That allows me to filter out all single occurences.
Then I wanted to work with an if formula to test if B2 is different from B1. If not, then I need to concatenate the value in column A with something. As I said, I can't seem to figure out how to dow this.

I hope somebody can help me.

Thank you in advance.