Hold up will happen when all enemies are knocked down. They all like pening on the ground.

Hit them with their weaknesses. Once all are down, they'll be in Hold-up. It's best when you can Baton pass between your characters to cycle between them to get to their weaknesses. Elemental weaknesses like Wind (Garu), Fire (Agi), Ice(Bufu) and Nuclear (Frei) can be dealt with by the first 4 team members you recruit in the game. Some attacks improves the chances of dealing "Critical" damage, and when that happens, your enemies will go down the same way as when you attack them for their weaknesses, irregardless if they're weak for that attack or not. Some skills also improves weakness susceptibility by "breaking" their resistance against that said skill (Fire, Ice, Wind, Nuclear), or making them more weak against them, improving the chances you get them to go down.

Also, assuming you wanna "hold them up" to persuade them to join you, read the tutorial about how to deal with the Personas, their conversation traits and which answer will entice them towards joining you. A lot of people miss these the first time around because they just wanna skip past the tutorial popup message to get straight to the Hold-up sequence. Once you've met enough of these Personas, you can already know from the style of their conversation to pick the right conversation option.

If you're holding them up to get them to give you money, there is a better way than that: