The kinds of heroes you can put into a squad is limited by your Cost Limit. Your Cost Limit is determined by your Squad Level, which increases as you gain XP or become a VIP. Becoming a VIP 7 and above raises your Cost Limit by 5 each time.
The more powerful a hero (Level, Star Ranking, Equipment), the higher its Cost. For example, you can’t put five Level 90 Heroes that have 13 Cost each in one squad when your Cost Limit is 50. You would at the very least need a Cost Limit of 65.
To specify the squad leader, tap “Change Leader” and tap the hero you wish to be the squad leader.
Tap “Change Position” and drag your heroes to the formation positions you want. Certain heroes, like Shilda, gain bonuses from being placed at certain parts of the formation. Refer to their Skills for information about this.