Who is allowed to join in the sacrament?

Answer 1

Only those who are Water baptized, legally married (If you are married), and walking in relationship with the Lord...

Answer 2

Eucharist, unlike communion in other denominations, is a little more serious, due to the Catholic belief in transubstantiation. This means they believe that the wine and bread used in communion transforms very literally into the blood and body of Jesus Christ. As such, the allowance of non-members to participate is going to depend on the particular priest delivering the communion. If you're not sure, ask the priest if you're allowed. Even if he says no, he'll still likely grant you a blessing, as everyone's allowed a blessing.

Answer 3

Generally, anyone who has been baptized.

It does differ from Denomination to Denomination.
Some require that you only believe in their God, others require you to be a member of the church, or be baptized.

Practically, in the Roman Catholic Church, there are certain tenets that should be followed (confession, and previously fasting was required). But there are very few cases of it being withheld or refused from anyone.

Answer 4

who is allowed to take the holy sacrement