You don't automatically have the ability to deploy groups at the start even though you get units that already have the "Group" ability. You get it later.
Then it depends on the stage whether or not you can deploy group. If you can deploy groups on the stage, then simply selecting one member of the group will automatically select everyone within the group. If not they'll be deployed individually as usual. There is no need to do anything "special" to deploy groups.
Also, on stages where you can deploy groups, there's a limit to how many you can bring. Usually 4 I think. --- http://ontlogy.wordpress.com/ "Have withstood pain to read many words."
Groups are set in stone, you don't actually build any. On the mission stage you will sometimes notice that a mission has a red GR in it. That means you can deploys groups(Max of 4).
The grouping system actually makes sense when you consider that most of the groups are units that usually worked together in their individual series i.e. the 08th ms team,the valkyries,votoms(Red Shoulders, Barcoff squad) etc.
Full Group List
Scopedog TC RS - Chirico & rest of Red Shoulders Scopedog TC ISS - Barcoff squad Scopedog RSC, Berserga WP, AT Fly - Chirico,Ru Shako & Vanilla Shield Liger, Command Wolf, Gustav MS - Ban, Irvine, Moonbay Blade Liger, Lightning Saix, Dibison - Ban, Irvine, Thomas Iron Kong Mk-II, Red Horn, Redler - Black, Rosso, Viola (DLC UNITS) Gun Sniper LS, Gun Sniper NS - Leela & Naomi(sp?) (DLC UNITS) Gundam, Guncannon, Guntank, G Fighter - Amuro, Kai, Hayato, Sayla Gundam Ez8, Ground Gundam, Ground Gundam/GM Head, Hovertruck - Shiro, Sanders, Karen, Michel NT-1 Alex, Zaku II Kai - Christina, Bernie L Gaim, Dseerd, Novel Dseerd - Daba, Amu, Leece Ingram Unit-1,Ingram Unit-2,Type-0 - Noa, Ohta, Kanuka Valkyries(Alto & Ozma have the Super & Armored Packs respectively) - Alto, Ozma, Michael, Luca
These are the only units that can deploy in groups and you can't pick and choose units within a group to go together. For Chirico, Ban, Amuro, & Daba they have extra units that can be added to the deploy list at the same time as their group however only one instance of a pilot can be out at once.
Example:The last stage of Ch4 has the Red Shoulders & Barcoff squad deployed, Chirico deploys with the Barcoff squad in his Scopedog TC ISS and the Red Shoulders deploy in their Scopedog TC Rs' minus Chirico's. In order to deploy him in his Scopdog TC RS, you have to put both groups back in White Base and next turn deploy the Red Shoulders first with Chirico & then the Barcoff squad who will be missing Chirico's Scopedog TC ISS. --- I lost my family, my friends, and everything in my life however I'll continue until I can see them again.