We anticipate most users to create their own meta units.
Here are some of the current ways you can do it but we hope to get feedback on other ways:
1. Player selects units, right-clicks on another unit to have it join that meta.
2. Player selects a unit and has units constructed and sent to it to join a unit.
3. Player drag selects a bunch of units, creates a control group and units not currently in a meta unit are put in one as best the game can (non ideal but better than nothing).
Other things we are considering but are not promising (scope cost) are being able to design meta units outside the game and then being able to, in game, select a unit and choose a meta unit design and have the types of units that have been considered show up as icons that the player can select and have built and/or unassociated units attach to.