You have several options available:
Use the member search to look for interesting XING members, colleagues or friends. If you are a premium member you can use the advanced member search which allows you to search individual profile fields.
Suggestions from XING
If you go to the "Expand network" tab in your contacts section, we'll show you some members you may know.
Address book synchronisation
Want to know which of your e-mail contacts use XING? We can show you by synchronising your address book. Just select your e-mail provider, enter your login details for your e-mail account, and we'll show you a list of people you can add to your network. You can do this with as many e-mail addresses as you like.
Browse groups and see if there are any of interest to you. Any groups you join are bound to have members with the same interests as you who you could then add to your network.
XING hosts a large selection of events. By signing up to one, you'll benefit both from the content of the event and from the networking effects it brings with it.
Premium only: Find members tab
Premium members can check out the Find members tab to view members with similar profiles.