If you like a specific Group on the Network, you can let them know via email.
For instructions on this, see this article about how to forward a message on the Network -- the same approach applies to letting friends know about a Group.
You also welcome to try sending out invites using the Network's "Send Invites" function using the instructions below. However, the software that runs that system is a bit unreliable.
To use the "Send Invites" function to send a message to your "Friends" on the Network to join a Group:
1) Go to the Group on the Network.
2) Find the "Send Invites" tab.
3) If you're the group's Administrator, you can send anyone on the Network an invite. If you're just a member of the group, you can only send your Network "Friends" an invite (helps to eliminate invasive invitations). Just start typing a person's name into the search field and wait for the system to make suggestions (which will load as a drop down list).
4) When you're finished selecting, be sure to click "Send Invites."