Login to the SRAM portal.
In the "Collaborations"-tab, click on collaboration that you want to invite somebody to:
When you have selected the collaboration, go to the "Members"-tab.
Click on "Invite members" and fill in the form.
With the "Invite" button on the bottom of the form you send an e-mail to the invited persons.
If they accept the invitation, they will become members of the collaboration.