Previously on Ultimate X-Men, Cable (who in the Ultimate universe is an older version of Wolverine) came from the future to kill Xavier. Bishop came from the future to stop him and help the X-Men. In the end Cable apparently blew up Xavier and left Bishop stranded in the present. Cyclops disbanded the X-Men, and Bishop decided to form his own team.
This is from Ultimate X-Men #80, by Robert Kirkman and Yanick Paquette.
At the end of #80, it's revealed that Xavier is still alive, and is in the future with Cable.
In Ultimate X-Men #82, by Kirkman and Pascal Alixe...
In #83, Bishop recruits Dazzler and Angel.
Then, in Ultimate X-Men #84, by Kirkman and Paquette...
Bishop recruits Wolverine.
And, after recruiting Psylocke...
In #85...
After Bishop helped her use her powers more offensively, Dazzler is able to take down three of the sentinels at once.
This is Stryfe. He's the leader of the Mutant Liberation Front, and doesn't have any connection to his 616-counterpart.
It's later revealed that Bishop is really working with Cable.
In Ultimate X-Men #90 Sinister heads to the Morlocks' tunnels so he can kill six more mutants (having already killed four in BKV's run) and bring back Apocalypse. The art is by Salvador Larroca.
Sinister is transformed into Apocalypse.
In Ultimate X-Men #92, Cable and Professor X return to the present.
Later...
In Ultimate X-Men #93, Kirkman's last issue, Phoenix defeats Apocalypse. The art is by Harvey Tolibao.
When Aron Coleite's run on Ultimate X-Men started next issue, Pyro was nowhere to be seen.