Mike Glover and Tu Lam both spent time with Delta Force after first coming up through the Green Berets, the usual route into that world.
Their careers crossed paths with The Unit while they were still on active duty. Most accounts have described them as augmentations brought in for particular missions or stretches rather than as formally assigned members, though Glover has rejected that framing.
In his talk with Shawn Ryan, Mike made it clear he saw himself as actually belonging to The Unit, not merely attached to it, even if his time there was fairly brief.