How to add user to multiple groups

I created two groups: calendar contributor & editor, so that the users in both groups can add & edit pages and add & edit the calendar (ajax event calendar). However, I have to choose one role for each user. If I choose calendar contributor, she will one be able to add & edit calendar. If I choose editor, she will only be able to add & edit pages. I would like the users in this group to be able to add & edit pages as well as the calendar.

http://wordpress.org/plugins/user-access-manager/