"Let's X to play football"
can I replace X with a verb (or phrase)?
I was thinking of teamup (or is it team-up?) but I think it is used when we are already split and we want to be one team.
After years of playing informal sports, I can virtually assure you that the most appropriate phrase (in the US) would be to "pick teams"
pick verb 2b : choose, select
also please note
pickup noun 6: a pickup game playing pickup
Technically, it would seem that forming teams would be more correct, as there may not be a leader with absolute authority who does the picking. However, I think a brief internet search of "pick teams" vs. "form teams" will confirm that the former is certainly more popular than the latter regardless of technical accuracy.
Neither of 'Let's split to play football', nor 'Let's split-up to play football' would work very well in the UK. When we were children (a long time ago) we would say 'Let's pick-up teams for football'. Two persons would be selected (don't ask me how) as captains and then each in turn would select one player. (You always felt dreadful if you were the last man standing).
I have just asked my 8-year old grandson who lives in Manchester which considers itself the world capital of football. (Though the denizens of Rio, Madrid, Barcelona, Milan, Munich and London, to say nothing of Liverpool and Glasgow, may disagree). He says that he and his friends say 'Let's make teams'. But in selecting the teams, he tells me, they will use a system called 'ghost and numbers'. One person is the 'ghost' and he stands with his back to the others who are each given a number. The ghost does not know which numbers apply to each person, and will shout out the numbers for each team. My grandson thinks this is 'fairer' than selecting two captains. And I am encouraged by the fact that two generations below me they still give priority to 'fairness'.