You can add external contacts to your buddy list to communicate with them. The following options exist:
invite contacts from another Open-Xchange hosting environment to your buddy list
temporarily invite a guest user
How to add a contact from another Open-Xchange hosting environment to your buddy list:
In the menu bar, click on Add buddy.
In the Add to buddylist window, enter the E-Mail address of the contact from another Open-Xchange hosting environment. Click on Invite.
If this contact is not already in your address book, it will be added automatically.
How to temporarily invite a guest user to a conversation:
In the toolbar, click on Invite guest.
Enter the E-Mail address of the guest user in the Invite guest window. Click on Invite.
An url for the guest account will be shown in the Guest user created window.