Group O Matic makes it easier for the raid leader to set up the group arrangement. It can save and restore what groups people are in.

It also has an Auto button, which can push everyone to the top, or group melee together, or split the group into the odd/even group numbers, or other things. What the Auto button does is set by the "auto-arrange template" menu.

You can exclude the groups at the end of the raid from being changed, so that you can use those groups for bench/standby players (click More > Settings > Exclude groups).

The addon will show up when you open the raid UI.

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This addon also has some manual /commands:

/rgs 1 2 will swap the entire group 1 with the entire group 2.
/rgm 1 2 will push everyone from group 1 into group 2, if possible.

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Things the Auto button can be set to do (click More and pick from the auto-arrange template menu):

1. Group melee/tanks, healers, and range with similar players.
2. Push everyone to the top of the group list.
3. Split the group into two halves, attempting to balance tanks, healers and melee DPS.
4. Group players by what map location they are at.