They might be given some fitness program to ensure they stay in shape but I doubt they will train with the squad immediately. The season, especially in Werner's case, just finished and Lampard would likely want them to take some time off, disconnect from football before coming back later. After all, Lampard has mentioned numerous times how Pulisic didn't have a summer break last year and that he had to be managed carefully in his first season in English football etc. Last thing he would want is Werner, Ziyech to get overworked before the new season even starts and then get tired later on, especially when next season is gonna be mightily congested.