Jürgen Moltmann believed that Non-Christians should be invited to the Lord's Supper, and that there should be no restrictions to the eucharist (commonly referred to as "fencing the table"). Wolfhart Pannenberg disagreed with Moltmann, and in his Systematic Theology Vol. III, Pannenberg criticized Moltmann's "eucharist community" as too broad of an invitation. 

Pannenberg has considerable agreement with Moltmann, and Pannenberg explains that the Lord's Supper should not be restricted from any baptized Christian, and the invitation should be extended to all people who seek fellowship and discipleship of Jesus. Pannenberg agrees that the Lord's Supper should never be restricted to members of a denomination, and he also explains that 1 Cor 11:27-32 may not be carelessly generalized to restrict anyone from the Lord's Table who seeks discipleship from Jesus and fellowship in the Eucharist. (Pannenberg explains this in a fascinating section preceding the following long quotation.)

However, Pannenberg believes Moltmann's invitation is too broad, because Moltmann places no restrictions whatsoever upon the Lord's Table. Pannenberg agrees with Moltmann that the invitation to the Lord's Supper may be extended to sinners and tax collectors, but Pannenberg believes that this invitation should be restricted to those sinners and tax collectors who seek discipleship from Jesus and seek fellowship.   

Pannenberg makes his argument for restricting invitations to the Lord's Supper to only those who seek discipleship and fellowship based upon a verse in Didache 9:5 "But let no one eat or drink of your Eucharist, unless they have been baptized into the name of the Lord; for concerning this also the Lord has said, 'Give not that which is holy to the dogs.'" [c.f. Matt 7:6]

Wolfhart Pannenberg explains his criticism of Moltmann's open invitation as follows in his Systematic Theology Vol. III,

The Lord's Supper is not the expression of a human fellowship that is set up in some other way. Nor is it an expression of the fellowship of a specific church. It is not the church's Supper but the Supper of the Lord of his church. The invitation of Jesus that the celebrant issues when he says the words of institution in Jesus' stead (in persona Christi) is for all his disciples. Hence we have to speak of an open invitation of Christ that no church on its own may restrict to its own historically particular fellowship. J. Moltmann has rightly stated that what has to be justified in the eyes of the Crucified is not the openness of this invitation but every restrictive measure passed by churches. Can there be any reasons to justify restriction?

In 1970 I myself required that all baptized Christians be admitted to communion so long as we can assume a desire for the fellowship with Jesus that the Lord's Supper imparts. Moltmann, however, goes a step further than this and rejects any restriction on admission to communion. The open invitation of the Crucified to his Supper extends even beyond the bounds of the Christian world to all nations, and to sinners and tax collectors first of all. But the restriction of admission to those in whom we may assume a desire for fellowship with Jesus follows from the nature of the Last Supper, and although this formulation may be intentionally broad, it has very definite implications.

Like the Last Supper on the night when Jesus was betrayed, the church's Lord's Supper is a meal for disciples that presupposes discipleship on the part of the participants. This fact is connected indeed with the constitution of the 'new covenant' at the Last Supper. The inclusion of tax collectors and sinners in the table fellowship of the earthly Jesus is continued inasmuch as in principle the company of disciples is open to all who want to be disciples of Jesus. Even in the case of tax collectors and sinners the table fellowship with Jesus expressed a willingness for fellowship with him and an acceptance of the rule of God that was breaking in in his person. The table fellowship of Jesus up to his parting from his disciples took place within the old covenant people to whom tax collectors and sinners also belonged. At the last Supper, however, Jesus established the fellowship of the new covenant. Those who seek fellowship with Jesus at his Supper have to want this fellowship. 

Sources:

1. Wolfhart Pannenberg.  Systematic Theology: Volume 3., Eerdmans, 1997, pp. 329–330.