UbiBlog has details on how participants in the Rainbow Six Siege beta have the opportunity to invite up to four friends to join them over the weekend:

If you’re an existing player on PC, head to www.rainbow6keys.ubi.com, and you can give up to four buddies full access to the game through Uplay – just enough to fill out your squad. From 3:01 p.m. PST on Friday, December 18, to 9:01 p.m. PST on Sunday, December 20, they can join you (or strike out on their own) in 5v5 multiplayer matches or co-op Terrorist Hunt runs. They can even play through the single-player Situations, which is a good idea anyway, as it’ll help them acclimate to Rainbow Six Siege’s unique pacing and Operator abilities.

If just playing with buddies isn’t enough of an incentive to bring your friends in, referrers will also be rewarded with a 24-hour Renown Booster (to be delivered shortly after the program ends) for each friend that redeems a key. You can send your friends keys right now, giving them the chance to preload through Uplay and be ready to breach at 3:01 p.m. PST on Friday. Best of all, if any “recruits” decide they want to keep playing after Sunday, they’ll be able to get a 25% discount on the full game from Uplay, and keep all the progress they’ve earned.