Aikidō is generally trained in pairs, one person plays the role of 受け, the attacker, the other person is the 取り, who receives the attack and does the technique. After some time, typically four times, Uke and Tori switch their roles. Depending on the dōjō, pairs switch either between techniques, or when the teacher says so.
The building up of pairs for training is supposed to be spontaneous, which means there is a lot of group and social dynamics in play: typically everybody would prefer to train with somebody more advanced than oneself, and some people, some styles are more compatible than other, some seek out higher ranking people actively, while other lower level persons tend to shy away, not daring to impose oneself. At the end of each technique, there is a subtle dance, as people place themselves so they can ask spontaneously their neighbour for the next technique.
As the number of people practicing is pretty random, there is often an odd number of people training, which means that one group in the dōjō where three people practice together. The protocol for the remaining single person is to join an existing pair, typically sitting in seiza and waiting for the pair to notice her. What I find interesting is the speed at which the practicing group notices the person waiting. Some immediately notice the waiting one, while others are so engrossed in the technique it takes them ages to acknowledge the third person, to the point that some other pair typically invites the third one.
The thing I started noticing is that people who don’t notice the third waiting person are also often the ones that get overwhelmed during 乱取り. Randori is the mock fight practice: tori is now attacked by one, two or more uke, typically on a fixed attack, and defends himself using a free technique. I find two things difficult in randori, first keeping my breathing in check, as it is quite easy to be overwhelmed by two attackers, the second is tactics, i.e. being aware of where the various uke are, and selecting a technique that places you in a good position relative to the other uke, i.e. not in the middle, not with them in your back, ideally with the falling uke between you and them.
So being aware of position of the second uke is very important, and so being able to notice such a person during regular training is in my opinion a good training for randori.