Agents will gain experience during missions with you and dispatch missions. Make sure that you recruit one from each of the 4 classes (Commando, Recon, Engineer, Support) and have one of each reach Rank 5. Sub-classes (backgrounds selected at recruitment) doesnt count.
With making sure to select each class agents, it will happen during the campaign progression without any extra effort. Will pop once the 4th agent reaches Rank 5.