You can tell your friends about a group by doing the following:
1. Open/Closed Groups
If you're the group's admin
Press "Add group member", displayed on the right side of the group page.
You can send the "Invitation URL" via messages to invite members.
If you're not the admin
You can just tell them the group page's URL.
When they click "Join", they'll be able to join the group.
* If joining the group requires Admin approval, they'll be able to join once approved by an admin.
2. Secret Groups
Only the Admin can add people to a secret group.
Press "Add group member", displayed on the right side of the group page.
You can send the "Invitation URL" via messages to invite members to bring them to the group page.
* Underage users and those with browsing restrictions cannot join R-18 groups.