There are various ways you can interact with other players in Darkfall Unholy Wars.
This can be accomplished directly, by approaching a player in the world, or indirectly, through the chat window or social window.
Interaction
Approaching a player in the world and holding down the Interact Key 
while aiming at them will present various options that can be selected through a radial wheel.
The options available are Trade, Invite to Party, Send friend request and Whisper. If you are in a clan and have sufficient permissions, an Invite to clan option will also be available.
Start TradeInvite to PartySend Friend Request WhisperInvite to Clan
Party
You can create a party by inviting any other player to it. You can invite players to a party by approaching them, holding down the use button and selecting the appropriate option from the radial menu.
You can also right click on player names in any chat window and select the Invite to party option.
Party Floating Markers
When you find yourself in a party, either one you created or were invited to, you will be able to see yours and the other party members names, health and primary school on the right side of the screen.
Here you can also see various effects that your party members have active. You will also be able to see the other party member locations on the map at all times.
You can leave a party at any point by right clicking on your name in the party list and selecting the leave party option.
If you were the one to create the party by inviting another player, you will be able to remove or promote players by right clicking on their name and selecting the appropriate option.
Keep in mind that when in a party, Prowess rewards for various accomplishments are split between party members. The player that performed the action gets half of the Prowess reward and the rest is split evenly among all party members.