Unfortunately it's currently not possible for a SWITCHengines user to directly add other users to a project.
For the moment, you need to send mail to firstname.lastname@example.org and ask us (the SWITCHengines admins) to do that for you - just mention the project and user name(s). We will usually do this within a few hours.
In the future, we'll try to find a way to delegate management of projects so that this becomes a "self service" operation. The standard authorization mechanisms of OpenStack aren't really powerful enough to allow this, but together with our colleagues from other research network operators we'll find a way.