Wait For Your Turn
You can elect to act later in the round, stepping in at any point to act. If you have elected to wait until another player's turn has come up, you will be able to act after they have taken their turn in the round.
Example: Turn order is player A, then B, then C. Player A decides to wait until player C has moved from cover, then take his shot. By waiting, the new turn order will be B, C then A.
You may want to speed up your play by designating one member of the group as the party leader and have him roll initiative for the whole group. His roll is added to REF scores of everyone in the group to determine when each member of the group will act.