might depend on the jobs they have ? i have 8 Sims 6 of them work jobs listed as follows and who i can join and who goes off on their on
Steve-works at the stadium -heads of on own ( there is no option to join)
Julia catering mixology branch -heads of on own ( there is no option to join)
Josue catering chef   -heads of on own ( there is no option to join)
the next three these ones i can send alone OR join BUT these three head out roughly at the same time i can only choose one of them to join so i could join Peter at his job today the next working day i can select to join Brittany the next working day Thomas  OR i can keep sending Peter and Brittany alone and join Thomas everyday to max out his career or just keep sending all of them alone 
Thomas police
Brittany hospital
Peter hospital