Usually, the easiest way is to copy the link to your group or project from your browser address bar and send it somehow to the people you want to invite. When they follow the link, they will see the microsite and the "Sign Up" (if they already have a [we_faq_organisation_name] profile) and "Register" (if they don't have one). Once they have registered or logged in, they will be taken back to your microsite and can request membership. Admins will be notified and can confirm the membership request.
Admins have another option for adding members. In the "Members" section, they can select already registered users that they want to add, or send an invitation email to people who are not yet registered. This option eliminates the step of users first applying for membership and an admin having to confirm it.
NEW: In the group or project settings, the “type of participation application” can be changed so that users do not have to apply for membership first, but can join directly.