This one took me about an hour to figure out, it's still flawed, I'll explain that in a bit.
In order for this to work, you'll need to put this in every Monster group. The only upside to that is if you want to limit where the summon can be used, that will be a great help.
Make a new skill, and name it. Change the type to "Switch", and make the MP consumption to your liking. We want it to turn on a switch, so let's click on the button next to "Turn ON Switch", and make a new trigger called "Summon: (your creature)", and make sure the Activation is only set to Battle.
Now make a new Hero, customize this Hero to the way you want your summoned person/creature to be like.
Now go to monster groups. Edit one to the way you want. Now in the trigger, check the box next to the top switch, and then the button with the "...", and set it to "Summon: (your creature)", and click ok. Now down below insert a new event, and set it to "Change Party" (that's 9 buttons down on the left side), and make sure the button for "Add member" is on, and for the specific Character, open the slide and scroll to your creature's name, and select that, then click "Ok". Then insert anotherSwitch operation, but make the "Summon Switch" OFF, otherwise the next battle you'll have the summon right away.
Now make a new page. Here is a limit that I cannot figure out. I want the summon for only a certain amount of time, so I'm trying to figure that out, but for now, make the trigger to "Monster [(whichever monster)]'s HP is between [0-(whatever number. the higher the number, the less help the summoned creature gives)]. and then down below insert "Change party members", and set the button to "Remove", and make sure your creature's name is listed in the box.
And that's that. I really want to do a time limit, but the "wait" command will freeze everything, which you don't want. I tried to figure out the turns commands, and I'm still working towards it.
There you go, a true summoning command, instead of an attack that looks like a summon.
In the summoned creatures options, you can set it to "AI Control" For a more authentic "summoned creature" instead of a controlled creature.