When creating a new PM, you need to specify the recipient of the message. I typically type the member name if I know it, rather than using the search feature.
When typing the recipient name, the recipient field is "active". When this field is active, the forum software provides a list of members that are on-line that are in your folders/friends list (not sure which or if both) that are currently on-line. These member are flagged with an "Add" label. The list and the "Add" labels are not normally visible to me when in the recipient field. They are further down on the screen.
If you hit the <enter> key to terminate the member name in the active recipient field, the software automatically adds the top member on the list to the recipient field. Due to the font of the added members, its not all that obvious to me that another name was added to the recipients.
This is a feature that I do not like, but some will find handy. However, now that I know what is happening, I can adjust to it, but I like to have more control over what happens when I am typing and hit <enter>.