"For the Son of Man is to come with his angels in the glory of his Father, and then he will repay everyone for what has been done.  Truly I tell you, there are some standing here who will not taste death before they see the Son of Man coming in his kingdom."

Verse 27 is a greatly condensed version of what will be the Last Judgment, explained more fully in Matthew 25.31-46.  Jesus says this after talking to the disciples about the necessity of suffering and self-denial for those who wish to follow and obey God (verses 16.21-26).  "Those who want to save their life will lose it, and those who lose their life for my sake will find it."  Jesus then mentions the Last Judgment as though to make very concrete that the finding and losing of lives will be a very real occurrence which will reach its final manifestation at the Last Judgment, when a person will go off either into eternal punishment or eternal glory.

Regarding verse 28, a commentary on Mark 9.1, an identical passage, states "The coming of the Kingdom of God with power does not seem to refer to the second, glorious coming of Jesus at the end of time (the Parousia); it may, rather, indicate the amazing spread of the Church in the lifetime of the Apostles.  Many of those present here will witness this." (The Navarre Bible, St. Mark, p. 127)  Taken together then these passages indicate that the repayment of a life lived either in obedience to God or out of lust for the world will have both a temporal aspect as well as a permanent, final state.  One begins either to lose one's life or to find one's life here, now, in the midst of daily affairs wherein "the kingdom of God has already begun."

All the holy host of angels will be present at the Final Judgment.  Why?  They have a part in the final act as witnesses because they have played a key part in the temporal act as witnesses of our lives.  They see all our goings and comings, inspire us on to acts of self-denial and cross-bearing and therefore witness before God on our obedience or disobedience.  Since they play such a part now that the kingdom has started but has not reached its fullness they will come all together as an innumerable host at the end of time when the Judge repays everyone for what has been done or not done.

If you want to deny yourself, take up your cross and follow Christ therefore (v. 24), listen closely to the promptings of the angels, most especially your own dearest guardian angel who is desirous that you reach the heights of virtue.  Think often of the angels.  Lean heavily upon them each day.  They are your witnesses before God and you will recognize those you either listened to or despised when you see them again at the Last Judgment.  They desire a favorable outcome for you far more than you do for yourself because they burn like the stars for love of God's human creatures - because we are his creatures along with them but most especially due to the Incarnation.