I was at a conference once and they told us about using clock partners to team up students in the class. The concept is that students walk around and set an appointment time with other students and record their appointment on their clock (example: I go to you and ask if you have a five o'clock open. You tell me yes and I write your name in for 5 p.m. and you write my name in your 5 p.m. slot). You can specify that you can only list a persons name once or you must have equal number of boys and girls to keep it fair.
I found a template on the internet and recreated it on my own (feel free to email me if you want me to send it to you). I had my students get their clock partners at the beginning of the year and they glued it to the back inside cover of their science notebook. If I need to group students I just say, "Get with your two o'clock partner" and they know what to do.
I don't use them as often as I should. I've been trying to correct this recently. The other day I had students get with their three o'clock partner to work on the two worksheets in the post below. I did not want that activity to take a long time and I figured if I paired students up that I could get it done quickly. As always you get the mismatched, "I dislike you" kind of pairing. I never change partners. Instead I say that they have the option of doing the entire activity by themselves, as long as they check their answers with each other at the end of the activity. They are also not allowed to ask me for help until they have asked their partner for help. That helps reduce the complaining.
As always students come and go throughout the year. If a person has a partner that has left the school they automatically get any new student to the class in that time slot...so always have students write names in pencil (although I can see middle and high school students erasing names to put their friends name for a called time slot so gauge that how you see fit).