COIN-OR runs a number of Google discussion groups. These include:
Coin-Announce – annoucements of general interest to the Operations Research and COIN-OR user community
Coin-Discuss – General discussion about COIN-OR activities and software
Coin-Voting-Members – A group for members of COIN-OR used for elections and other similar activities
We also have (or will have soon) discussion groups on Github for each of the COIN-OR projects. For example, the COIN-OR CBC discussion group is at https://github.com/coin-or/Cbc/discussions