Yes, surely without the positive participation of employees the team building activities will not give desired results. The purpose of team building is to develop harmony, better working and problem solving skills, ability to work as a team, willingness to play for a team and to develop better understanding the nature and personalities of the other team members. Therefore if some person is not involved actively in the team building activities how such events can bear a sweet fruit? That's why active participation of employees is must in team building activities. What you can do is to tell benefits of such events to the employees, i.e. You may tell them take it's an opportunity to increase your social circle, ask them to let's enjoy and play in such events, you may also tell them that you'll give prizes to the winners etc. You may also tell them that having friendly relations will help them in solving their problems in easy and tension-free manner. It's also possible that some employees may be feeling shyness and nervousness while interacting with higher authorities in an informal way. To remove shyness of junior staff managers should take initiative initially and may opt for performing first before asking for juniors to come forward.