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.