Ordinarily, the grammatically correct versions of the sentences you presented are as follows:
"The user was added to the group." (Not "The user is added to the group.")
"The following sections were modified:" (Not "The following sections are modified:)"
Because the action was done and completed in the past, the passive-voice past tense of the verb ("was added," "were modified") is used instead of the active-voice present tense form ("is added," "are modified").
However, the linking verb can be used in the present tense ("is," "are") if we modify the passive-voice sentence into one denoting a state or condition, as in the following:
"The user is an added element to the group."
"The following sections are modified versions of the original."
In such constructions, the phrase that follows the linking verb functions as an adjective describing the subject. There is no action involved in the sentence.
Now the question is: Wouldn't it be possible to use the active-voice, present-tense constructions at all?
Yes, there's a special case. It is when the speaker or writer is making the statement at the very moment of utterance or the act of writing as part of a set of instructions. For instance,
TV TALK SHOW HOST: "This is how e-groups is organized. First, the provider team is formed. Then the user is added to the group..."
or:
A FIRST-PERSON WRITTEN NARRATIVE USING THE PRESENT TENSE:
"Let me describe to you how e-groups are organized in Alpha Company. First, the provider team is formed. Then the user is added to the group..."
This, of course, is precisely the nature of the technical documentation statements that you provided. They are online instructions in real time, so the present-tense, active-voice construction is perfectly acceptable--in fact is the scrupulously correct usage--in each case.
